fix: DNG handling when reading data from filepath directly (#151)

* fix: DNG handling

* fix: Clarify docs

* fix: Clarify docs 2

* fix: Refactor

* fix: Bump version

Author: tmathern

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "c2pa-python"
-version = "0.16.0"
+version = "0.17.0"
 requires-python = ">=3.10"
 description = "Python bindings for the C2PA Content Authenticity Initiative (CAI) library"
 readme = { file = "README.md", content-type = "text/markdown" }

src/c2pa/c2pa.py

@@ -654,6 +654,33 @@ def load_settings(settings: str, format: str = "json") -> None:
     return result
 
 
+def _get_mime_type_from_path(path: Union[str, Path]) -> str:
+    """Attempt to guess the MIME type from a file path (with extension).
+
+    Args:
+        path: File path as string or Path object
+
+    Returns:
+        MIME type string
+
+    Raises:
+        C2paError.NotSupported: If MIME type cannot be determined
+    """
+    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
+        return "image/dng"
+    else:
+        mime_type = mimetypes.guess_type(str(path))[0]
+        if not mime_type:
+            raise C2paError.NotSupported(
+                f"Could not determine MIME type for file: {path}")
+        return mime_type
+
+
 def read_ingredient_file(
         path: Union[str, Path], data_dir: Union[str, Path]) -> str:
     """Read a file as C2PA ingredient.
@@ -1214,7 +1241,11 @@ def __init__(self,
         """Create a new Reader.
 
         Args:
-            format_or_path: The format or path to read from
+            format_or_path: The format or path to read from.
+              The stream API (params format and an open stream) is
+              the recommended way to use the Reader. For paths, we
+              will attempt to guess the mimetype of the source
+              file based on the extension.
             stream: Optional stream to read from (Python stream-like object)
             manifest_data: Optional manifest data in bytes
 
@@ -1231,12 +1262,8 @@ 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]
 
-            if not mime_type:
-                raise C2paError.NotSupported(
-                    f"Could not determine MIME type for file: {path}")
+            mime_type = _get_mime_type_from_path(path)
 
             if mime_type not in Reader.get_supported_mime_types():
                 raise C2paError.NotSupported(
@@ -2269,7 +2296,9 @@ def sign_file(self,
         """Sign a file and write the signed data to an output file.
 
         Args:
-            source_path: Path to the source file
+            source_path: Path to the source file. We will attempt
+              to guess the mimetype of the source file based on
+              the extension.
             dest_path: Path to write the signed file to
             signer: The signer to use
 
@@ -2279,11 +2308,8 @@ 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]
-        if not mime_type:
-            raise C2paError.NotSupported(
-                f"Could not determine MIME type for file: {source_path}")
+
+        mime_type = _get_mime_type_from_path(source_path)
 
         try:
             # Open source file and destination file, then use the sign method

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")