diff --git a/ErsatzTV.Application/Artworks/ArtworkContentTypeModel.cs b/ErsatzTV.Application/Artworks/ArtworkContentTypeModel.cs
index f2d75c62a..393af4d46 100644
--- a/ErsatzTV.Application/Artworks/ArtworkContentTypeModel.cs
+++ b/ErsatzTV.Application/Artworks/ArtworkContentTypeModel.cs
@@ -1,5 +1,5 @@
-using System.Net;
 using ErsatzTV.Core.Domain;
+using ErsatzTV.Core.Images;
 
 namespace ErsatzTV.Application.Artworks;
 
@@ -11,7 +11,14 @@ public record ArtworkContentTypeModel(string Path, string ContentType)
 
     public bool HasContentType => !string.IsNullOrWhiteSpace(ContentType);
 
-    public string UrlWithContentType => string.IsNullOrWhiteSpace(ContentType)
-        ? Path
-        : $"{Path}?contentType={WebUtility.UrlEncode(ContentType)}";
+    // The artwork serve routes now sniff the content type from the stored file and no longer honor a
+    // client-supplied ?contentType= (issue #283 — that reflection was the stored-XSS sink), so the
+    // directly-usable URL is just the path.
+    public string UrlWithContentType => Path;
+
+    // Defense-in-depth: never persist a content type outside the image allow-list, so a value that
+    // slipped in via the {path, contentType} JSON DTOs can't later be reflected anywhere. The serve
+    // path derives the type from the file regardless; this only keeps stored metadata honest.
+    public ArtworkContentTypeModel Sanitized() =>
+        ImageContentTypes.IsAccepted(ContentType) ? this : this with { ContentType = string.Empty };
 }
diff --git a/ErsatzTV.Application/Artworks/Commands/UploadArtwork.cs b/ErsatzTV.Application/Artworks/Commands/UploadArtwork.cs
index e27c60b37..84da5d211 100644
--- a/ErsatzTV.Application/Artworks/Commands/UploadArtwork.cs
+++ b/ErsatzTV.Application/Artworks/Commands/UploadArtwork.cs
@@ -9,5 +9,5 @@ namespace ErsatzTV.Application.Artworks;
 ///     landing it in the same on-disk cache the Blazor UI uses (via <c>IImageCache</c>),
 ///     so the returned path is equivalent to a Blazor-uploaded image.
 /// </summary>
-public record UploadArtwork(Stream Stream, string ContentType, ArtworkKind ArtworkKind)
+public record UploadArtwork(Stream Stream, ArtworkKind ArtworkKind)
     : IRequest<Either<BaseError, ArtworkUploadResponseModel>>;
diff --git a/ErsatzTV.Application/Artworks/Commands/UploadArtworkHandler.cs b/ErsatzTV.Application/Artworks/Commands/UploadArtworkHandler.cs
index d8623baea..c2ab153d9 100644
--- a/ErsatzTV.Application/Artworks/Commands/UploadArtworkHandler.cs
+++ b/ErsatzTV.Application/Artworks/Commands/UploadArtworkHandler.cs
@@ -1,22 +1,13 @@
 using ErsatzTV.Core;
 using ErsatzTV.Core.Api.Artwork;
 using ErsatzTV.Core.Domain;
+using ErsatzTV.Core.Images;
 using ErsatzTV.Core.Interfaces.Images;
 
 namespace ErsatzTV.Application.Artworks;
 
 public class UploadArtworkHandler : IRequestHandler<UploadArtwork, Either<BaseError, ArtworkUploadResponseModel>>
 {
-    // png/jpeg/gif/webp are all decoded by SkiaSharp and read by FFmpeg, matching the
-    // formats the Blazor logo/watermark upload already accepts. Format expansion is ersatztv#66.
-    private static readonly System.Collections.Generic.HashSet<string> AcceptedContentTypes = new(StringComparer.OrdinalIgnoreCase)
-    {
-        "image/png",
-        "image/jpeg",
-        "image/gif",
-        "image/webp"
-    };
-
     private readonly IImageCache _imageCache;
 
     public UploadArtworkHandler(IImageCache imageCache) => _imageCache = imageCache;
@@ -25,15 +16,31 @@ public class UploadArtworkHandler : IRequestHandler<UploadArtwork, Either<BaseEr
         UploadArtwork request,
         CancellationToken cancellationToken)
     {
-        string contentType = (request.ContentType ?? string.Empty).Trim();
-        if (!AcceptedContentTypes.Contains(contentType))
+        // Buffer the upload so we can sniff its true format before storing it. The request body is
+        // already bounded by the Kestrel MaxRequestBodySize / the controller's size check, so this
+        // is a bounded read.
+        byte[] bytes;
+        await using (var buffer = new MemoryStream())
         {
-            return BaseError.New(
-                $"Unsupported image content type '{contentType}'; supported types are: {string.Join(", ", AcceptedContentTypes)}");
+            await request.Stream.CopyToAsync(buffer, cancellationToken);
+            bytes = buffer.ToArray();
         }
 
+        // Derive the content type from the actual bytes, never from the client-declared value
+        // (issue #283 — a spoofed image/png header let a <script> payload be stored and later served
+        // as HTML). A payload that isn't a supported raster image is rejected here.
+        Option<string> maybeContentType = ImageContentTypes.DetectContentType(bytes);
+        if (maybeContentType.IsNone)
+        {
+            return BaseError.New(
+                $"Uploaded file is not a supported image; supported types are: {string.Join(", ", ImageContentTypes.Accepted)}");
+        }
+
+        string contentType = maybeContentType.IfNone(string.Empty);
+
+        using var toCache = new MemoryStream(bytes, writable: false);
         Either<BaseError, string> maybeFileName = await _imageCache.SaveArtworkToCache(
-            request.Stream,
+            toCache,
             request.ArtworkKind);
 
         return maybeFileName.Map(fileName => new ArtworkUploadResponseModel(
diff --git a/ErsatzTV.Application/Images/Queries/GetCachedImagePath.cs b/ErsatzTV.Application/Images/Queries/GetCachedImagePath.cs
index 1fa0d9774..d44f8e7c8 100644
--- a/ErsatzTV.Application/Images/Queries/GetCachedImagePath.cs
+++ b/ErsatzTV.Application/Images/Queries/GetCachedImagePath.cs
@@ -3,6 +3,6 @@ using ErsatzTV.Core.Domain;
 
 namespace ErsatzTV.Application.Images;
 
-public record GetCachedImagePath(string FileName, ArtworkKind ArtworkKind, string ContentType, int? MaxHeight = null)
+public record GetCachedImagePath(string FileName, ArtworkKind ArtworkKind, int? MaxHeight = null)
     : IRequest<
         Either<BaseError, CachedImagePathViewModel>>;
diff --git a/ErsatzTV.Application/Images/Queries/GetCachedImagePathHandler.cs b/ErsatzTV.Application/Images/Queries/GetCachedImagePathHandler.cs
index e6575f45b..380ac3b4f 100644
--- a/ErsatzTV.Application/Images/Queries/GetCachedImagePathHandler.cs
+++ b/ErsatzTV.Application/Images/Queries/GetCachedImagePathHandler.cs
@@ -1,6 +1,7 @@
 using CliWrap;
 using ErsatzTV.Core;
 using ErsatzTV.Core.Domain;
+using ErsatzTV.Core.Images;
 using ErsatzTV.Core.Interfaces.FFmpeg;
 using ErsatzTV.Core.Interfaces.Images;
 using ErsatzTV.Core.Interfaces.Repositories;
@@ -95,9 +96,13 @@ public class
             }
             else
             {
-                mimeType = !string.IsNullOrWhiteSpace(request.ContentType)
-                    ? request.ContentType
-                    : MimeTypes.GetMimeTypeFromFile(cachePath).Name;
+                // Always derive the type from the stored file — never from a client-supplied value
+                // (issue #283 — the old ?contentType= reflection was the stored-XSS sink). Clamp the
+                // sniffed type to the image allow-list so a file whose bytes are not an accepted image
+                // (a legacy cache entry poisoned before the upload sniff landed, or a hypothetical
+                // polyglot) is served as a non-renderable download, never as HTML/script.
+                string sniffed = MimeTypes.GetMimeTypeFromFile(cachePath)?.Name;
+                mimeType = ImageContentTypes.IsAccepted(sniffed) ? sniffed : "application/octet-stream";
             }
 
             return new CachedImagePathViewModel(cachePath, mimeType);
diff --git a/ErsatzTV.Core.Tests/Images/ImageContentTypesTests.cs b/ErsatzTV.Core.Tests/Images/ImageContentTypesTests.cs
new file mode 100644
index 000000000..902b908db
--- /dev/null
+++ b/ErsatzTV.Core.Tests/Images/ImageContentTypesTests.cs
@@ -0,0 +1,60 @@
+using System.Text;
+using ErsatzTV.Core.Images;
+using NUnit.Framework;
+using Shouldly;
+using SkiaSharp;
+
+namespace ErsatzTV.Core.Tests.Images;
+
+[TestFixture]
+public class ImageContentTypesTests
+{
+    [TestCase(SKEncodedImageFormat.Png, "image/png")]
+    [TestCase(SKEncodedImageFormat.Jpeg, "image/jpeg")]
+    [TestCase(SKEncodedImageFormat.Webp, "image/webp")]
+    public void DetectContentType_Should_Sniff_Real_Image_Bytes(SKEncodedImageFormat format, string expected)
+    {
+        byte[] bytes = EncodeTinyImage(format);
+
+        Option<string> detected = ImageContentTypes.DetectContentType(bytes);
+
+        detected.IfNone(string.Empty).ShouldBe(expected);
+    }
+
+    [Test]
+    public void DetectContentType_Should_Reject_Non_Image_Payload()
+    {
+        // The stored-XSS payload from issue #283 — script bytes that a spoofed content type would
+        // otherwise have let through.
+        byte[] html = Encoding.UTF8.GetBytes("<script>alert(document.domain)</script>");
+
+        ImageContentTypes.DetectContentType(html).IsNone.ShouldBeTrue();
+    }
+
+    [Test]
+    public void DetectContentType_Should_Return_None_For_Empty_Or_Null()
+    {
+        ImageContentTypes.DetectContentType([]).IsNone.ShouldBeTrue();
+        ImageContentTypes.DetectContentType(null).IsNone.ShouldBeTrue();
+    }
+
+    [TestCase("image/png", true)]
+    [TestCase("image/jpeg", true)]
+    [TestCase("image/gif", true)]
+    [TestCase("image/webp", true)]
+    [TestCase("IMAGE/PNG", true)]
+    [TestCase("text/html", false)]
+    [TestCase("image/svg+xml", false)]
+    [TestCase("", false)]
+    [TestCase(null, false)]
+    public void IsAccepted_Should_Allow_Only_Raster_Image_Types(string contentType, bool expected) =>
+        ImageContentTypes.IsAccepted(contentType).ShouldBe(expected);
+
+    private static byte[] EncodeTinyImage(SKEncodedImageFormat format)
+    {
+        using var bitmap = new SKBitmap(2, 2);
+        using SKImage image = SKImage.FromBitmap(bitmap);
+        using SKData data = image.Encode(format, 100);
+        return data.ToArray();
+    }
+}
diff --git a/ErsatzTV.Core/Images/ImageContentTypes.cs b/ErsatzTV.Core/Images/ImageContentTypes.cs
new file mode 100644
index 000000000..26acd47c2
--- /dev/null
+++ b/ErsatzTV.Core/Images/ImageContentTypes.cs
@@ -0,0 +1,64 @@
+using SkiaSharp;
+
+namespace ErsatzTV.Core.Images;
+
+/// <summary>
+///     The single source of truth for which image content types the artwork surfaces accept, plus a
+///     byte-sniffing detector used to derive the true content type from an upload's actual contents
+///     rather than trusting the client-declared value (issue #283 — stored XSS via a spoofed
+///     content type). png/jpeg/gif/webp are the formats SkiaSharp decodes and FFmpeg reads, matching
+///     what the artwork upload has always accepted; format expansion is tracked by ersatztv#66.
+/// </summary>
+public static class ImageContentTypes
+{
+    private static readonly IReadOnlyDictionary<SKEncodedImageFormat, string> FormatContentTypes =
+        new Dictionary<SKEncodedImageFormat, string>
+        {
+            [SKEncodedImageFormat.Png] = "image/png",
+            [SKEncodedImageFormat.Jpeg] = "image/jpeg",
+            [SKEncodedImageFormat.Gif] = "image/gif",
+            [SKEncodedImageFormat.Webp] = "image/webp"
+        };
+
+    public static readonly IReadOnlySet<string> Accepted =
+        new System.Collections.Generic.HashSet<string>(FormatContentTypes.Values, StringComparer.OrdinalIgnoreCase);
+
+    /// <summary>
+    ///     Sniffs the true image content type from <paramref name="bytes" /> using SkiaSharp's codec
+    ///     detection. Only the image header is parsed to identify the format — pixels are never
+    ///     decoded here, so a crafted decompression-bomb image cannot be triggered by this call.
+    ///     Returns None for anything that is not one of the accepted raster formats (including
+    ///     non-image payloads such as HTML/script uploaded under a spoofed content type).
+    /// </summary>
+    public static Option<string> DetectContentType(byte[] bytes)
+    {
+        if (bytes is null || bytes.Length == 0)
+        {
+            return Option<string>.None;
+        }
+
+        try
+        {
+            using SKData data = SKData.CreateCopy(bytes);
+            using SKCodec codec = SKCodec.Create(data);
+            if (codec is null)
+            {
+                return Option<string>.None;
+            }
+
+            return FormatContentTypes.TryGetValue(codec.EncodedFormat, out string contentType)
+                ? contentType
+                : Option<string>.None;
+        }
+        catch (Exception)
+        {
+            return Option<string>.None;
+        }
+    }
+
+    /// <summary>
+    ///     True when <paramref name="contentType" /> is one of the accepted image content types.
+    /// </summary>
+    public static bool IsAccepted(string contentType) =>
+        !string.IsNullOrWhiteSpace(contentType) && Accepted.Contains(contentType.Trim());
+}
diff --git a/ErsatzTV.Tests/Application/Artworks/ArtworkContentTypeModelTests.cs b/ErsatzTV.Tests/Application/Artworks/ArtworkContentTypeModelTests.cs
new file mode 100644
index 000000000..2c61d764e
--- /dev/null
+++ b/ErsatzTV.Tests/Application/Artworks/ArtworkContentTypeModelTests.cs
@@ -0,0 +1,39 @@
+using ErsatzTV.Application.Artworks;
+using NUnit.Framework;
+using Shouldly;
+
+namespace ErsatzTV.Tests.Application.Artworks;
+
+[TestFixture]
+public class ArtworkContentTypeModelTests
+{
+    [Test]
+    public void Sanitized_Should_Keep_An_Allow_Listed_Content_Type()
+    {
+        var model = new ArtworkContentTypeModel("iptv/logos/abc", "image/png");
+
+        model.Sanitized().ShouldBe(model);
+    }
+
+    [TestCase("text/html")]
+    [TestCase("application/javascript")]
+    [TestCase("image/svg+xml")]
+    [TestCase("")]
+    public void Sanitized_Should_Blank_A_Non_Allow_Listed_Content_Type(string contentType)
+    {
+        var model = new ArtworkContentTypeModel("iptv/logos/abc", contentType);
+
+        ArtworkContentTypeModel sanitized = model.Sanitized();
+
+        // Path is preserved; only the untrusted content type is neutralized.
+        sanitized.Path.ShouldBe("iptv/logos/abc");
+        sanitized.ContentType.ShouldBe(string.Empty);
+    }
+
+    [Test]
+    public void UrlWithContentType_Should_Be_Just_The_Path()
+    {
+        // The serve route no longer honors ?contentType= (issue #283), so the URL carries no query.
+        new ArtworkContentTypeModel("iptv/logos/abc", "image/png").UrlWithContentType.ShouldBe("iptv/logos/abc");
+    }
+}
diff --git a/ErsatzTV.Tests/Application/Artworks/UploadArtworkHandlerTests.cs b/ErsatzTV.Tests/Application/Artworks/UploadArtworkHandlerTests.cs
index 3a4792a90..35c8e05fd 100644
--- a/ErsatzTV.Tests/Application/Artworks/UploadArtworkHandlerTests.cs
+++ b/ErsatzTV.Tests/Application/Artworks/UploadArtworkHandlerTests.cs
@@ -14,6 +14,11 @@ namespace ErsatzTV.Tests.Application.Artworks;
 [TestFixture]
 public class UploadArtworkHandlerTests
 {
+    // A minimal valid 1x1 PNG. The handler derives the content type from bytes like these, never
+    // from a client-declared value (issue #283), so the tests exercise the real sniffer.
+    private static readonly byte[] PngBytes = Convert.FromBase64String(
+        "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAQAAAC1HAwCAAAAC0lEQVR42mNk+M8AAAMCAoAB/QwAAAAASUVORK5CYII=");
+
     private IImageCache _imageCache = null!;
     private UploadArtworkHandler _handler = null!;
 
@@ -25,17 +30,18 @@ public class UploadArtworkHandlerTests
     }
 
     [Test]
-    public async Task Handle_Should_Return_Logo_Path_With_Iptv_Logos_Prefix()
+    public async Task Handle_Should_Store_Logo_And_Return_Sniffed_ContentType()
     {
         _imageCache.SaveArtworkToCache(Arg.Any<Stream>(), ArtworkKind.Logo)
-            .Returns(Right<BaseError, string>("abc123.png"));
+            .Returns(Right<BaseError, string>("abc123"));
 
-        using var stream = new MemoryStream();
+        using var stream = new MemoryStream(PngBytes);
         Either<BaseError, ArtworkUploadResponseModel> result =
-            await _handler.Handle(new UploadArtwork(stream, "image/png", ArtworkKind.Logo), CancellationToken.None);
+            await _handler.Handle(new UploadArtwork(stream, ArtworkKind.Logo), CancellationToken.None);
 
         ArtworkUploadResponseModel response = RightOf(result);
-        response.Path.ShouldBe("iptv/logos/abc123.png");
+        response.Path.ShouldBe("iptv/logos/abc123");
+        // Content type comes from the actual PNG bytes, not from anything the client said.
         response.ContentType.ShouldBe("image/png");
     }
 
@@ -43,25 +49,41 @@ public class UploadArtworkHandlerTests
     public async Task Handle_Should_Return_Bare_File_Name_For_Watermark()
     {
         _imageCache.SaveArtworkToCache(Arg.Any<Stream>(), ArtworkKind.Watermark)
-            .Returns(Right<BaseError, string>("def456.webp"));
+            .Returns(Right<BaseError, string>("def456"));
 
-        using var stream = new MemoryStream();
+        using var stream = new MemoryStream(PngBytes);
         Either<BaseError, ArtworkUploadResponseModel> result = await _handler.Handle(
-            new UploadArtwork(stream, "image/webp", ArtworkKind.Watermark),
+            new UploadArtwork(stream, ArtworkKind.Watermark),
             CancellationToken.None);
 
-        RightOf(result).Path.ShouldBe("def456.webp");
+        RightOf(result).Path.ShouldBe("def456");
     }
 
     [Test]
-    public async Task Handle_Should_Reject_Unsupported_Content_Type()
+    public async Task Handle_Should_Reject_Non_Image_Payload_Spoofed_As_Image()
+    {
+        // The XSS chain: a <script> payload uploaded under a spoofed image content type. The handler
+        // decodes the bytes, sees no supported image, and rejects it before anything is stored.
+        byte[] htmlBytes = System.Text.Encoding.UTF8.GetBytes("<script>alert(document.domain)</script>");
+        using var stream = new MemoryStream(htmlBytes);
+
+        Either<BaseError, ArtworkUploadResponseModel> result = await _handler.Handle(
+            new UploadArtwork(stream, ArtworkKind.Logo),
+            CancellationToken.None);
+
+        LeftOf(result).Value.ShouldContain("not a supported image");
+        await _imageCache.DidNotReceive().SaveArtworkToCache(Arg.Any<Stream>(), Arg.Any<ArtworkKind>());
+    }
+
+    [Test]
+    public async Task Handle_Should_Reject_Empty_Upload()
     {
         using var stream = new MemoryStream();
         Either<BaseError, ArtworkUploadResponseModel> result = await _handler.Handle(
-            new UploadArtwork(stream, "image/bmp", ArtworkKind.Logo),
+            new UploadArtwork(stream, ArtworkKind.Logo),
             CancellationToken.None);
 
-        LeftOf(result).Value.ShouldContain("Unsupported image content type");
+        LeftOf(result).Value.ShouldContain("not a supported image");
         await _imageCache.DidNotReceive().SaveArtworkToCache(Arg.Any<Stream>(), Arg.Any<ArtworkKind>());
     }
 
@@ -71,9 +93,9 @@ public class UploadArtworkHandlerTests
         _imageCache.SaveArtworkToCache(Arg.Any<Stream>(), ArtworkKind.Logo)
             .Returns(Left<BaseError, string>(BaseError.New("disk full")));
 
-        using var stream = new MemoryStream();
+        using var stream = new MemoryStream(PngBytes);
         Either<BaseError, ArtworkUploadResponseModel> result = await _handler.Handle(
-            new UploadArtwork(stream, "image/png", ArtworkKind.Logo),
+            new UploadArtwork(stream, ArtworkKind.Logo),
             CancellationToken.None);
 
         LeftOf(result).Value.ShouldBe("disk full");
diff --git a/ErsatzTV.Tests/Controllers/ArtworkUploadControllerTests.cs b/ErsatzTV.Tests/Controllers/ArtworkUploadControllerTests.cs
index 8d2f44457..0c507ddba 100644
--- a/ErsatzTV.Tests/Controllers/ArtworkUploadControllerTests.cs
+++ b/ErsatzTV.Tests/Controllers/ArtworkUploadControllerTests.cs
@@ -113,12 +113,13 @@ public class ArtworkUploadControllerTests
 
         var created = result.ShouldBeOfType<CreatedResult>();
         created.StatusCode.ShouldBe(201);
-        created.Location.ShouldBe("/iptv/logos/abc.png?contentType=image%2Fpng");
+        // No ?contentType= — the serve route sniffs the stored file (issue #283).
+        created.Location.ShouldBe("/iptv/logos/abc.png");
         created.Value.ShouldBeOfType<ArtworkUploadResponseModel>()
             .Path.ShouldBe("iptv/logos/abc.png");
 
         await _mediator.Received(1).Send(
-            Arg.Is<UploadArtwork>(c => c.ArtworkKind == ArtworkKind.Logo && c.ContentType == "image/png"),
+            Arg.Is<UploadArtwork>(c => c.ArtworkKind == ArtworkKind.Logo),
             Arg.Any<CancellationToken>());
     }
 
@@ -133,7 +134,7 @@ public class ArtworkUploadControllerTests
         IActionResult result = await _controller.Upload(file, "watermark", CancellationToken.None);
 
         var created = result.ShouldBeOfType<CreatedResult>();
-        created.Location.ShouldBe("/artwork/watermarks/def.webp?contentType=image%2Fwebp");
+        created.Location.ShouldBe("/artwork/watermarks/def.webp");
 
         await _mediator.Received(1).Send(
             Arg.Is<UploadArtwork>(c => c.ArtworkKind == ArtworkKind.Watermark),
diff --git a/ErsatzTV/Controllers/Api/ArtworkUploadController.cs b/ErsatzTV/Controllers/Api/ArtworkUploadController.cs
index 487820a73..aead9f0f1 100644
--- a/ErsatzTV/Controllers/Api/ArtworkUploadController.cs
+++ b/ErsatzTV/Controllers/Api/ArtworkUploadController.cs
@@ -44,11 +44,11 @@ public class ArtworkUploadController(IMediator mediator) : ControllerBase
 
         await using Stream stream = file.OpenReadStream();
         Either<BaseError, ArtworkUploadResponseModel> result = await mediator.Send(
-            new UploadArtwork(stream, file.ContentType, artworkKind),
+            new UploadArtwork(stream, artworkKind),
             cancellationToken);
 
         return result.ToCreatedResult(
-            value => LocationFor(artworkKind, value.Path, value.ContentType),
+            value => LocationFor(artworkKind, value.Path),
             value => value);
     }
 
@@ -70,16 +70,13 @@ public class ArtworkUploadController(IMediator mediator) : ControllerBase
         }
     }
 
-    // Both GetImage (IptvController) and GetWatermark (ArtworkController) require a contentType
-    // query param to serve the cached file, so the Location header must carry it too.
-    private static string LocationFor(ArtworkKind artworkKind, string path, string contentType)
-    {
-        string encodedContentType = Uri.EscapeDataString(contentType);
-        return artworkKind switch
+    // The serve routes (GetImage / GetWatermark) sniff the content type from the stored file, so the
+    // Location no longer carries a ?contentType= (issue #283 — that reflection was the XSS sink).
+    private static string LocationFor(ArtworkKind artworkKind, string path) =>
+        artworkKind switch
         {
             // logo paths already carry the servable prefix ("iptv/logos/{file}")
-            ArtworkKind.Logo => $"/{path}?contentType={encodedContentType}",
-            _ => $"/artwork/watermarks/{path}?contentType={encodedContentType}"
+            ArtworkKind.Logo => $"/{path}",
+            _ => $"/artwork/watermarks/{path}"
         };
-    }
 }
diff --git a/ErsatzTV/Controllers/Api/Requests/CreateChannelFromLineupRequest.cs b/ErsatzTV/Controllers/Api/Requests/CreateChannelFromLineupRequest.cs
index 60df675b4..f901b72e6 100644
--- a/ErsatzTV/Controllers/Api/Requests/CreateChannelFromLineupRequest.cs
+++ b/ErsatzTV/Controllers/Api/Requests/CreateChannelFromLineupRequest.cs
@@ -32,7 +32,7 @@ public record CreateChannelFromLineupRequest(
             Number,
             Group,
             Categories,
-            Logo,
+            (Logo ?? ArtworkContentTypeModel.None).Sanitized(),
             IsEnabled,
             ShowInEpg,
             TemplateId,
diff --git a/ErsatzTV/Controllers/Api/Requests/CreateChannelRequest.cs b/ErsatzTV/Controllers/Api/Requests/CreateChannelRequest.cs
index c1fafb073..fb2d64e3a 100644
--- a/ErsatzTV/Controllers/Api/Requests/CreateChannelRequest.cs
+++ b/ErsatzTV/Controllers/Api/Requests/CreateChannelRequest.cs
@@ -45,7 +45,7 @@ public record CreateChannelRequest(
             Categories,
             FFmpegProfileId,
             SlugSeconds,
-            Logo,
+            (Logo ?? ArtworkContentTypeModel.None).Sanitized(),
             StreamSelectorMode,
             StreamSelector,
             PreferredAudioLanguageCode,
diff --git a/ErsatzTV/Controllers/Api/Requests/CreateWatermarkRequest.cs b/ErsatzTV/Controllers/Api/Requests/CreateWatermarkRequest.cs
index a5898e4d7..d5dbd80be 100644
--- a/ErsatzTV/Controllers/Api/Requests/CreateWatermarkRequest.cs
+++ b/ErsatzTV/Controllers/Api/Requests/CreateWatermarkRequest.cs
@@ -27,7 +27,7 @@ public record CreateWatermarkRequest(
     public CreateWatermark ToCommand() =>
         new(
             Name,
-            new ArtworkContentTypeModel(Image ?? string.Empty, ImageContentType ?? string.Empty),
+            new ArtworkContentTypeModel(Image ?? string.Empty, ImageContentType ?? string.Empty).Sanitized(),
             Mode,
             ImageSource,
             Location,
diff --git a/ErsatzTV/Controllers/Api/Requests/UpdateChannelRequest.cs b/ErsatzTV/Controllers/Api/Requests/UpdateChannelRequest.cs
index f028084e6..1cce873f2 100644
--- a/ErsatzTV/Controllers/Api/Requests/UpdateChannelRequest.cs
+++ b/ErsatzTV/Controllers/Api/Requests/UpdateChannelRequest.cs
@@ -46,7 +46,7 @@ public record UpdateChannelRequest(
             Categories,
             FFmpegProfileId,
             SlugSeconds,
-            Logo,
+            (Logo ?? ArtworkContentTypeModel.None).Sanitized(),
             StreamSelectorMode,
             StreamSelector,
             PreferredAudioLanguageCode,
diff --git a/ErsatzTV/Controllers/Api/Requests/UpdateWatermarkRequest.cs b/ErsatzTV/Controllers/Api/Requests/UpdateWatermarkRequest.cs
index d999d42c1..ad2f49653 100644
--- a/ErsatzTV/Controllers/Api/Requests/UpdateWatermarkRequest.cs
+++ b/ErsatzTV/Controllers/Api/Requests/UpdateWatermarkRequest.cs
@@ -28,7 +28,7 @@ public record UpdateWatermarkRequest(
         new(
             id,
             Name,
-            new ArtworkContentTypeModel(Image ?? string.Empty, ImageContentType ?? string.Empty),
+            new ArtworkContentTypeModel(Image ?? string.Empty, ImageContentType ?? string.Empty).Sanitized(),
             Mode,
             ImageSource,
             Location,
diff --git a/ErsatzTV/Controllers/ArtworkController.cs b/ErsatzTV/Controllers/ArtworkController.cs
index 17d714cb9..2e8c9afc5 100644
--- a/ErsatzTV/Controllers/ArtworkController.cs
+++ b/ErsatzTV/Controllers/ArtworkController.cs
@@ -65,7 +65,7 @@ public class ArtworkController : ControllerBase
     {
         Either<BaseError, CachedImagePathViewModel> cachedImagePath =
             await _mediator.Send(
-                new GetCachedImagePath(fileName, ArtworkKind.Poster, string.Empty, 440),
+                new GetCachedImagePath(fileName, ArtworkKind.Poster, 440),
                 cancellationToken);
         return cachedImagePath.Match<IActionResult>(
             Left: _ => new NotFoundResult(),
@@ -75,13 +75,11 @@ public class ArtworkController : ControllerBase
     [HttpGet("/artwork/watermarks/{fileName}")]
     public async Task<IActionResult> GetWatermark(
         string fileName,
-        [FromQuery]
-        string contentType,
         CancellationToken cancellationToken)
     {
         Either<BaseError, CachedImagePathViewModel> cachedImagePath =
             await _mediator.Send(
-                new GetCachedImagePath(fileName, ArtworkKind.Watermark, contentType),
+                new GetCachedImagePath(fileName, ArtworkKind.Watermark),
                 cancellationToken);
         return cachedImagePath.Match<IActionResult>(
             Left: _ => new NotFoundResult(),
@@ -92,7 +90,7 @@ public class ArtworkController : ControllerBase
     public async Task<IActionResult> GetFanArt(string fileName, CancellationToken cancellationToken)
     {
         Either<BaseError, CachedImagePathViewModel> cachedImagePath =
-            await _mediator.Send(new GetCachedImagePath(fileName, ArtworkKind.FanArt, string.Empty), cancellationToken);
+            await _mediator.Send(new GetCachedImagePath(fileName, ArtworkKind.FanArt), cancellationToken);
         return cachedImagePath.Match<IActionResult>(
             Left: _ => new NotFoundResult(),
             Right: r => new PhysicalFileResult(r.FileName, r.MimeType));
@@ -167,7 +165,7 @@ public class ArtworkController : ControllerBase
     {
         Either<BaseError, CachedImagePathViewModel> cachedImagePath =
             await _mediator.Send(
-                new GetCachedImagePath(fileName, ArtworkKind.Thumbnail, string.Empty, 220),
+                new GetCachedImagePath(fileName, ArtworkKind.Thumbnail, 220),
                 cancellationToken);
         return cachedImagePath.Match<IActionResult>(
             Left: _ => new NotFoundResult(),
diff --git a/ErsatzTV/Controllers/IptvController.cs b/ErsatzTV/Controllers/IptvController.cs
index d70ed71af..8868577d7 100644
--- a/ErsatzTV/Controllers/IptvController.cs
+++ b/ErsatzTV/Controllers/IptvController.cs
@@ -283,10 +283,10 @@ public class IptvController : StreamingControllerBase
     [HttpGet("iptv/logos/{fileName}")]
     [HttpHead("iptv/logos/{fileName}.jpg")]
     [HttpGet("iptv/logos/{fileName}.jpg")]
-    public async Task<IActionResult> GetImage(string fileName, [FromQuery] string contentType)
+    public async Task<IActionResult> GetImage(string fileName)
     {
         Either<BaseError, CachedImagePathViewModel> cachedImagePath =
-            await _mediator.Send(new GetCachedImagePath(fileName, ArtworkKind.Logo, contentType));
+            await _mediator.Send(new GetCachedImagePath(fileName, ArtworkKind.Logo));
         return cachedImagePath.Match<IActionResult>(
             Left: _ => new NotFoundResult(),
             Right: r => new PhysicalFileResult(r.FileName, r.MimeType));
diff --git a/ErsatzTV/Program.cs b/ErsatzTV/Program.cs
index 2bfcae1f0..02e1ada54 100644
--- a/ErsatzTV/Program.cs
+++ b/ErsatzTV/Program.cs
@@ -172,6 +172,11 @@ public class Program
                     }
 
                     options.AddServerHeader = false;
+
+                    // Bound the request body to the configured upload cap so an oversized (e.g.
+                    // multipart artwork) body is rejected as it's read, before it is buffered
+                    // (issue #283 S9 — the controller's file.Length check only fired after binding).
+                    options.Limits.MaxRequestBodySize = (long)SystemEnvironment.MaximumUploadMb * 1024 * 1024;
                 })
                 .UseContentRoot(BasePath))
             .UseSerilog();
diff --git a/docs/api-conventions.md b/docs/api-conventions.md
index eefbe63f7..705964b6f 100644
--- a/docs/api-conventions.md
+++ b/docs/api-conventions.md
@@ -216,6 +216,22 @@ Do **not** reuse the Application-layer Mappers used by Blazor (e.g. `MediaCards`
 mappers) for new API DTOs — those still return the old Blazor-convention relative paths. Map from
 the domain/VM directly and root the path yourself, following the PR #181 pattern.
 
+### 4a. Artwork content type is sniffed, never client-supplied (issue #283)
+
+The uploaded-artwork surfaces (channel logo, watermark) must never trust a client-declared content
+type — doing so was a stored-XSS chain (upload `<script>` as `image/png`, serve it back as
+`text/html`). The contract:
+- **Upload**: `POST /api/artwork/uploads` derives the content type from the actual bytes via
+  `ErsatzTV.Core/Images/ImageContentTypes.DetectContentType` (SkiaSharp header sniff — no full
+  decode), rejecting non-images 422. That helper (`Accepted` set + `IsAccepted`) is the **single
+  source of truth** for which image types are allowed — reuse it, don't re-list content types.
+- **Serve**: the image routes derive the served `Content-Type` from the stored file; there is **no**
+  `?contentType=` query parameter. Never add one back — a client must not be able to choose the
+  `Content-Type` of an unauthenticated response.
+- **Persisted `{path, contentType}` DTOs** (logo/watermark) run their content type through
+  `ArtworkContentTypeModel.Sanitized()` before storage, blanking anything outside the allow-list.
+See `docs/decisions.md` 2026-07-12 (#283) for the fuller rationale.
+
 ## 5. OpenAPI regeneration — commit both generated artifacts
 
 After any controller/DTO change:
diff --git a/docs/decisions.md b/docs/decisions.md
index 73fce2a61..8b5d72584 100644
--- a/docs/decisions.md
+++ b/docs/decisions.md
@@ -942,6 +942,42 @@ The larger #197 posture (fail-closed writes, sensitive-read auth tier, CORS lock
 the OpenAPI security scheme) is decomposed into #280–#289 with the phased roadmap on #197; those PRs will
 append their own decisions here as they land.
 
+## 2026-07-12 — Artwork content-type is sniffed, never reflected (#283, S4/S9 stored XSS)
+
+The artwork upload/serve path trusted client-supplied content types at both ends, giving a stored-XSS
+chain on **unauthenticated** GET sinks: upload `<script>` bytes declared `image/png` →
+`GET /iptv/logos/{hash}?contentType=text/html` served them as HTML in the ErsatzTV origin. The #279
+`nosniff` header is not a fix here — the server was *explicitly declaring* `text/html`, which the browser
+honors regardless of `nosniff`. The trust was the bug; the fix removes it at both ends.
+
+- **Upload derives the content type from the bytes, never the declared value.** `UploadArtworkHandler`
+  buffers the (size-bounded) upload and calls `ErsatzTV.Core/Images/ImageContentTypes.DetectContentType`,
+  which uses SkiaSharp's `SKCodec` to identify the format from the image header only — pixels are **not**
+  decoded, so this can't be turned into a decompression-bomb vector. A payload that isn't one of the
+  accepted raster formats (png/jpeg/gif/webp) is rejected 422; the declared `Content-Type` is no longer
+  read at all (the field was dropped from the `UploadArtwork` command).
+- **Serve sniffs the stored file; the `?contentType=` reflection is gone.** `GetCachedImagePath` no longer
+  carries a `ContentType`, and `GetImage` (`/iptv/logos`) / `GetWatermark` (`/artwork/watermarks`) dropped
+  their `[FromQuery] contentType` binding. `GetCachedImagePathHandler` always derives the MIME type from the
+  file (`MimeTypes.GetMimeTypeFromFile`) and **clamps it to the image allow-list** (`ImageContentTypes.IsAccepted`),
+  serving `application/octet-stream` for anything else — so a file whose bytes are not an accepted image (a
+  legacy cache entry poisoned before the upload sniff landed, or a hypothetical polyglot) is a non-renderable
+  download, never HTML/script. The removal is **structural** — there is no longer any request path that lets a
+  client choose the served `Content-Type`. `ArtworkContentTypeModel.UrlWithContentType` now returns the bare path, and the SPA
+  watermark/logo previews no longer append the query.
+- **Defense-in-depth on the persisted JSON DTOs.** The `{path, contentType}` bodies (channel logo, watermark)
+  run their content type through `ArtworkContentTypeModel.Sanitized()`, which blanks anything outside the
+  image allow-list before it is stored — so a stale/hostile value can't be reflected by any future code path
+  even though the serve route already ignores it.
+- **S9 upload-size DoS.** Kestrel `Limits.MaxRequestBodySize` is now set from `ETV_MAXIMUM_UPLOAD_MB`, so an
+  oversized body is rejected as it is read rather than only after the controller's post-binding `file.Length`
+  check (kept as the friendly-error backstop). This is a global bound; the app has no other large inbound
+  body (streaming is outbound GET).
+
+`ImageContentTypes` is the single source of truth for the accepted image types (the allow-list previously
+duplicated in `UploadArtworkHandler`). Both serve sinks are `[ApiExplorerSettings(IgnoreApi = true)]`, so
+none of this changes the OpenAPI document.
+
 ## 2026-07-12 — Fail-closed API auth + sensitive-read tier + CORS/ForwardedHeaders lockdown (#197 Bundle A, PR #292)
 
 Phase-1 of the #197 remediation — the auth posture that must land before any remote exposure.
diff --git a/web/src/api/watermarks.test.ts b/web/src/api/watermarks.test.ts
index f205040d2..119ba968d 100644
--- a/web/src/api/watermarks.test.ts
+++ b/web/src/api/watermarks.test.ts
@@ -79,8 +79,8 @@ describe('watermarks api client', () => {
     expect(fetchMock).toHaveBeenCalledWith('/api/watermarks/9', expect.objectContaining({ method: 'DELETE' }));
   });
 
-  it('watermarkImageUrl encodes the content type query', () => {
-    expect(watermarkImageUrl('abc.png', 'image/png')).toBe('/artwork/watermarks/abc.png?contentType=image%2Fpng');
+  it('watermarkImageUrl builds a path-only URL (no reflected ?contentType=)', () => {
+    // The serve route sniffs the stored file; the client no longer supplies a content type (#283).
     expect(watermarkImageUrl('abc.png')).toBe('/artwork/watermarks/abc.png');
   });
 });
diff --git a/web/src/api/watermarks.ts b/web/src/api/watermarks.ts
index 9156e8c91..e7a26127a 100644
--- a/web/src/api/watermarks.ts
+++ b/web/src/api/watermarks.ts
@@ -25,13 +25,11 @@ export function deleteWatermark(id: number): Promise<void> {
   return request<void>(`/api/watermarks/${id}`, { method: 'DELETE' });
 }
 
-// Preview URL for a custom watermark image. Mirrors the Blazor editor
-// (WatermarkEditor.razor) which renders `artwork/watermarks/{path}?contentType=...` via
-// ArtworkContentTypeModel.UrlWithContentType.
-export function watermarkImageUrl(path: string, contentType?: null | string): string {
-  return contentType
-    ? `/artwork/watermarks/${path}?contentType=${encodeURIComponent(contentType)}`
-    : `/artwork/watermarks/${path}`;
+// Preview URL for a custom watermark image. The serve route sniffs the content type from the
+// stored file and no longer honors a client-supplied `?contentType=` (issue #283 — that reflection
+// was the stored-XSS sink), so the URL is just the path.
+export function watermarkImageUrl(path: string): string {
+  return `/artwork/watermarks/${path}`;
 }
 
 export function messageFromWatermarkError(error: unknown, fallback = 'Unable to load watermarks'): string {
diff --git a/web/src/screens/WatermarksScreen.tsx b/web/src/screens/WatermarksScreen.tsx
index e9472f72b..c655a05c5 100644
--- a/web/src/screens/WatermarksScreen.tsx
+++ b/web/src/screens/WatermarksScreen.tsx
@@ -530,7 +530,7 @@ function WatermarkEditor({ mode }: { mode: { id: number; kind: 'edit' } | { from
             {custom && draft.image && (
               <img
                 alt="Watermark preview"
-                src={watermarkImageUrl(draft.image, draft.imageContentType)}
+                src={watermarkImageUrl(draft.image)}
                 style={{ maxHeight: 60, maxWidth: 200, objectFit: 'contain' }}
               />
             )}