Add SOF5 and SOF6 to the unsupported jpeg markers

src/ImageSharp/Formats/Jpeg/JpegConstants.cs

@@ -183,6 +183,16 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             /// </summary>
             public const byte SOF3 = 0xC3;
 
+            /// <summary>
+            /// Start of Frame marker, differential, Huffman coding, Differential sequential DCT.
+            /// </summary>
+            public const byte SOF5 = 0xC5;
+
+            /// <summary>
+            /// Start of Frame marker, differential, Huffman coding, Differential progressive DCT.
+            /// </summary>
+            public const byte SOF6 = 0xC6;
+
             /// <summary>
             /// Start of Frame marker, differential lossless, Huffman coding.
             /// </summary>

src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs

@@ -247,6 +247,14 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
                             this.ProcessStartOfFrameMarker(stream, remaining, fileMarker, metadataOnly);
                             break;
 
+                        case JpegConstants.Markers.SOF5:
+                            JpegThrowHelper.ThrowNotSupportedException("Decoding jpeg files with differential sequential DCT is not supported.");
+                            break;
+
+                        case JpegConstants.Markers.SOF6:
+                            JpegThrowHelper.ThrowNotSupportedException("Decoding jpeg files with differential progressive DCT is not supported.");
+                            break;
+
                         case JpegConstants.Markers.SOF3:
                         case JpegConstants.Markers.SOF7:
                             JpegThrowHelper.ThrowNotSupportedException("Decoding lossless jpeg files is not supported.");