fix: DNG handling

Author: tmathern

src/c2pa/c2pa.py

@@ -1231,8 +1231,22 @@ def __init__(self,
             # If we don't get a stream as param:
             # Create a stream from the file path in format_or_path
             path = str(format_or_path)
-            mime_type = mimetypes.guess_type(
-                path)[0]
+
+            # Extract file extension:
+            # path_obj.suffix returns the extension including
+            # the dot (e.g., ".jpg", ".png").
+            # If no extension exists, suffix returns empty string,
+            # so file_extension will be ""
+            path_obj = Path(path)
+            file_extension = path_obj.suffix.lower() if path_obj.suffix else ""
+
+            if file_extension == ".dng":
+                # mimetypes guesses the wrong type for dng,
+                # so we bypass it and set the correct type
+                mime_type = "image/dng"
+            else:
+                mime_type = mimetypes.guess_type(
+                    path)[0]
 
             if not mime_type:
                 raise C2paError.NotSupported(
@@ -2279,8 +2293,22 @@ def sign_file(self,
         Raises:
             C2paError: If there was an error during signing
         """
-        # Get the MIME type from the file extension
-        mime_type = mimetypes.guess_type(str(source_path))[0]
+        # Extract file extension:
+        # path_obj.suffix returns the extension
+        # including the dot (e.g., ".jpg", ".png")
+        # If no extension exists, suffix returns empty string,
+        # so file_extension will be ""
+        file_extension = ""
+        path_obj = Path(source_path)
+        file_extension = path_obj.suffix.lower() if path_obj.suffix else ""
+
+        if file_extension == ".dng":
+            # mimetypes guesses the wrong type for dng,
+            # so we bypass it and set the correct type
+            mime_type = "image/dng"
+        else:
+            mime_type = mimetypes.guess_type(str(source_path))[0]
+
         if not mime_type:
             raise C2paError.NotSupported(
                 f"Could not determine MIME type for file: {source_path}")

tests/fixtures/C.dng

@@ -126,9 +126,6 @@ def test_reader_streams_with_nested(self):
     def test_reader_close_cleanup(self):
         with open(self.testPath, "rb") as file:
             reader = Reader("image/jpeg", file)
-            # Store references to internal objects
-            reader_ref = reader._reader
-            stream_ref = reader._own_stream
             # Close the reader
             reader.close()
             # Verify all resources are cleaned up
@@ -144,6 +141,23 @@ def test_resource_to_stream_on_closed_reader(self):
         with self.assertRaises(Error):
             reader.resource_to_stream("", io.BytesIO(bytearray()))
 
+    def test_read_dng_from_stream(self):
+        test_path = os.path.join(self.data_dir, "C.dng")
+        with open(test_path, "rb") as file:
+            file_content = file.read()
+
+        with Reader("dng", io.BytesIO(file_content)) as reader:
+            # Just run and verify there is no crash
+            json.loads(reader.json())
+
+    def test_read_dng_file_from_path(self):
+        test_path = os.path.join(self.data_dir, "C.dng")
+
+        # Create reader with the file content
+        with Reader(test_path) as reader:
+            # Just run and verify there is no crash
+            json.loads(reader.json())
+
     def test_read_all_files(self):
         """Test reading C2PA metadata from all files in the fixtures/files-for-reading-tests directory"""
         reading_dir = os.path.join(self.data_dir, "files-for-reading-tests")