Only throw for multi SOF when fully decoding.

src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs

@@ -812,6 +812,11 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         {
             if (this.Frame != null)
             {
+                if (metadataOnly)
+                {
+                    return;
+                }
+
                 JpegThrowHelper.ThrowInvalidImageContentException("Multiple SOF markers. Only single frame jpegs supported.");
             }
 

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

@@ -32,6 +32,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
                 TestImages.Jpeg.Issues.ExifGetString750Load,
                 TestImages.Jpeg.Issues.ExifGetString750Transform,
                 TestImages.Jpeg.Issues.BadSubSampling1076,
+                TestImages.Jpeg.Issues.IdentifyMultiFrame1211,
 
                 // LibJpeg can open this despite the invalid density units.
                 TestImages.Jpeg.Issues.Fuzz.ArgumentOutOfRangeException825B,

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

@@ -38,6 +38,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
             { true, TestImages.Jpeg.Baseline.Jpeg400, 8, false, false },
             { true, TestImages.Jpeg.Baseline.Snake, 24, true, true },
             { true, TestImages.Jpeg.Baseline.Jpeg420Exif, 24, true, false },
+            { true, TestImages.Jpeg.Issues.IdentifyMultiFrame1211, 24, true, true },
         };
 
         public static readonly TheoryData<string, int, int, PixelResolutionUnit> RatioFiles =

tests/ImageSharp.Tests/TestImages.cs

@@ -229,6 +229,7 @@ namespace SixLabors.ImageSharp.Tests
                 public const string IncorrectResize1006 = "Jpg/issues/issue1006-incorrect-resize.jpg";
                 public const string ExifResize1049 = "Jpg/issues/issue1049-exif-resize.jpg";
                 public const string BadSubSampling1076 = "Jpg/issues/issue-1076-invalid-subsampling.jpg";
+                public const string IdentifyMultiFrame1211 = "Jpg/issues/issue-1221-identify-multi-frame.jpg";
 
                 public static class Fuzz
                 {