use folder.ext when poster.ext is not found for movies or shows (#96)

ErsatzTV.Core/Metadata/MovieFolderScanner.cs

@@ -210,6 +210,14 @@ namespace ErsatzTV.Core.Metadata
                     ext => new[] { $"{segment}.{ext}", Path.GetFileNameWithoutExtension(path) + $"-{segment}.{ext}" })
                 .Map(f => Path.Combine(folder, f));
             Option<string> result = possibleMoviePosters.Filter(p => _localFileSystem.FileExists(p)).HeadOrNone();
+            if (result.IsNone && artworkKind == ArtworkKind.Poster)
+            {
+                IEnumerable<string> possibleFolderPosters = ImageFileExtensions.Collect(
+                        ext => new[] { $"folder.{ext}" })
+                    .Map(f => Path.Combine(folder, f));
+                result = possibleFolderPosters.Filter(p => _localFileSystem.FileExists(p)).HeadOrNone();
+            }
+
             return result;
         }
     }

ErsatzTV.Core/Metadata/TelevisionFolderScanner.cs

@@ -319,15 +319,16 @@ namespace ErsatzTV.Core.Metadata
 
         private Option<string> LocateArtworkForShow(string showFolder, ArtworkKind artworkKind)
         {
-            string segment = artworkKind switch
+            string[] segments = artworkKind switch
             {
-                ArtworkKind.Poster => "poster",
-                ArtworkKind.FanArt => "fanart",
+                ArtworkKind.Poster => new[] { "poster", "folder" },
+                ArtworkKind.FanArt => new[] { "fanart" },
                 _ => throw new ArgumentOutOfRangeException(nameof(artworkKind))
             };
 
             return ImageFileExtensions
-                .Map(ext => $"{segment}.{ext}")
+                .Map(ext => segments.Map(segment => $"{segment}.{ext}"))
+                .Flatten()
                 .Map(f => Path.Combine(showFolder, f))
                 .Filter(s => _localFileSystem.FileExists(s))
                 .HeadOrNone();

ErsatzTV.Core/Plex/PlexTelevisionLibraryScanner.cs

@@ -131,7 +131,7 @@ namespace ErsatzTV.Core.Plex
                         result.IsUpdated = true;
                     }
                 }
-                
+
                 foreach (Studio studio in existingMetadata.Studios
                     .Filter(s => incomingMetadata.Studios.All(s2 => s2.Name != s.Name))
                     .ToList())