Add handler parsing

src/ImageSharp/Formats/Heic/Heic4CharCode.cs

@@ -198,4 +198,9 @@ public enum Heic4CharCode : uint
     /// </summary>
     uri  = 0x75726920U,
 
+    /// <summary>
+    /// Picture handler type.
+    /// </summary>
+    pict = 0x70696374U,
+
 }

src/ImageSharp/Formats/Heic/Heic4CharCode.tt

@@ -44,6 +44,7 @@
         "cdsc", "Content Description",
         "mime", "MIME type",
         "uri ", "URI",
+        "pict", "Picture handler type",
     };
 #>
 

src/ImageSharp/Formats/Heic/HeicDecoderCore.cs

@@ -192,21 +192,17 @@ internal sealed class HeicDecoderCore : IImageDecoderInternals
                 case Heic4CharCode.pitm:
                     this.ParsePrimaryItem(stream, length);
                     break;
-                case Heic4CharCode.dinf:
-                    SkipBox(stream, length);
-                    break;
                 case Heic4CharCode.hdlr:
-                    SkipBox(stream, length);
-                    break;
-                case Heic4CharCode.idat:
-                    SkipBox(stream, length);
+                    this.ParseHandler(stream, length);
                     break;
                 case Heic4CharCode.iloc:
                     this.ParseItemLocation(stream, length);
                     break;
+                case Heic4CharCode.dinf:
+                case Heic4CharCode.idat:
                 case Heic4CharCode.grpl:
                 case Heic4CharCode.ipro:
-                    // TODO: Implement
+                    // Silently skip these boxes.
                     SkipBox(stream, length);
                     break;
                 default:
@@ -215,6 +211,19 @@ internal sealed class HeicDecoderCore : IImageDecoderInternals
         }
     }
 
+    private void ParseHandler(BufferedReadStream stream, long boxLength)
+    {
+        Span<byte> buffer = this.ReadIntoBuffer(stream, boxLength);
+
+        // Only read the handler type, to check if this is not a movie file.
+        int bytesRead = 8;
+        Heic4CharCode handlerType = (Heic4CharCode)BinaryPrimitives.ReadUInt32BigEndian(buffer[bytesRead..]);
+        if (handlerType != Heic4CharCode.pict)
+        {
+            throw new ImageFormatException("Not a picture file.");
+        }
+    }
+
     private void ParseItemInfo(BufferedReadStream stream, long boxLength)
     {
         Span<byte> buffer = this.ReadIntoBuffer(stream, boxLength);