Handle incorrect colorspace metadata.

src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs

@@ -401,15 +401,14 @@ private JpegColorSpace DeduceJpegColorSpace()
 
             if (this.ComponentCount == 3)
             {
-                if (this.adobe.Equals(default) || this.adobe.ColorTransform == JpegConstants.Adobe.ColorTransformYCbCr)
-                {
-                    return JpegColorSpace.YCbCr;
-                }
-
-                if (this.adobe.ColorTransform == JpegConstants.Adobe.ColorTransformUnknown)
+                if (!this.adobe.Equals(default) && this.adobe.ColorTransform == JpegConstants.Adobe.ColorTransformUnknown)
                 {
                     return JpegColorSpace.RGB;
                 }
+
+                // Some images are poorly encoded and contain incorrect colorspace transform metadata.
+                // We ignore that and always fall back to the default colorspace.
+                return JpegColorSpace.YCbCr;
             }
 
             if (this.ComponentCount == 4)
@@ -419,7 +418,7 @@ private JpegColorSpace DeduceJpegColorSpace()
                     : JpegColorSpace.Cmyk;
             }
 
-            JpegThrowHelper.ThrowImageFormatException($"Unsupported color mode. Max components 4; found {this.ComponentCount}");
+            JpegThrowHelper.ThrowImageFormatException($"Unsupported color mode. Supported component counts 1, 3, and 4; found {this.ComponentCount}");
             return default;
         }
 

tests/ImageSharp.Tests/Formats/Jpg/JpegDecoderTests.Images.cs

@@ -31,9 +31,12 @@ public partial class JpegDecoderTests
                 TestImages.Jpeg.Issues.ExifGetString750Load,
                 TestImages.Jpeg.Issues.ExifGetString750Transform,
 
-                // LibJpeg can open this despite the invalid desity units.
+                // LibJpeg can open this despite the invalid density units.
                 TestImages.Jpeg.Issues.Fuzz.ArgumentOutOfRangeException825B,
 
+                // LibJpeg can open this despite incorrect colorspace metadata.
+                TestImages.Jpeg.Issues.IncorrectColorspace855,
+
                 // High depth images
                 TestImages.Jpeg.Baseline.Testorig12bit,
             };

tests/ImageSharp.Tests/TestImages.cs

@@ -175,6 +175,7 @@ public static class Issues
                 public const string ExifGetString750Transform = "Jpg/issues/issue750-exif-tranform.jpg";
                 public const string ExifGetString750Load = "Jpg/issues/issue750-exif-load.jpg";
                 public const string IncorrectQuality845 = "Jpg/issues/Issue845-Incorrect-Quality99.jpg";
+                public const string IncorrectColorspace855 = "Jpg/issues/issue855-incorrect-colorspace.jpg";
 
                 public static class Fuzz
                 {