Refactor MIME type detection

isic/ingest/models/accession.py

@@ -295,8 +295,7 @@ def generate_blob(self):
         try:
             with self.original_blob.open("rb") as original_blob_stream:
                 blob_mime_type = guess_mime_type(original_blob_stream, self.original_blob_name)
-            blob_major_mime_type = blob_mime_type.partition("/")[0]
-            if blob_major_mime_type != "image":
+            if blob_mime_type.major != "image":
                 raise InvalidBlobError(  # noqa: TRY301
                     f'Blob has a non-image MIME type: "{blob_mime_type}"'
                 )

isic/ingest/tests/test_utils_mime.py

@@ -1,6 +1,6 @@
 import pathlib
 
-from isic.ingest.utils.mime import guess_mime_type
+from isic.ingest.utils.mime import MimeType, guess_mime_type
 
 data_dir = pathlib.Path(__file__).parent / "data"
 
@@ -11,7 +11,7 @@ def test_utils_mime_guess_mime_type_consistent(caplog):
     with file_path.open("rb") as stream:
         mime_type = guess_mime_type(stream, file_path.name)
 
-    assert mime_type == "image/jpeg"
+    assert mime_type == MimeType("image/jpeg")
     assert not any("Inconsistent MIME types" in msg for msg in caplog.messages)
 
 
@@ -21,7 +21,7 @@ def test_utils_mime_guess_mime_type_inconsistent(caplog):
     with file_path.open("rb") as stream:
         mime_type = guess_mime_type(stream, "ISIC_0000000.gif")
 
-    assert mime_type == "image/jpeg"
+    assert mime_type == MimeType("image/jpeg")
     message = next((msg for msg in caplog.messages if "Inconsistent MIME types" in msg), None)
     assert message
     assert '"image/gif"' in message

isic/ingest/utils/mime.py

@@ -1,41 +1,55 @@
+from dataclasses import dataclass
 import logging
 import mimetypes
 import shutil
 import tempfile
 from typing import IO
 
-import magic
+from magic import Magic
 
 logger = logging.getLogger(__name__)
 
 
-def guess_mime_type(content: IO[bytes], source_filename: str | None = None) -> str:
+@dataclass
+class MimeType:
+    major: str
+    minor: str
+
+    def __init__(self, mime_type: str) -> None:
+        self.major, _, self.minor = mime_type.partition("/")
+
+    def __str__(self) -> str:
+        return f"{self.major}/{self.minor}"
+
+
+def guess_mime_type(content: IO[bytes], source_filename: str | None = None) -> MimeType:
     """
     Guess the MIME type of a file, based on its content.
 
     An optional `filename` can be provided, to provide extra context for guessing.
     """
-    m = magic.Magic(mime=True)
+    magic = Magic(mime=True)
 
     # This initial seek is just defensive
     content.seek(0)
-    with tempfile.SpooledTemporaryFile() as file_stream:
-        # Copy blob_stream into a SpooledTemporaryFile so it can be used by magic,
+    with tempfile.TemporaryFile() as file_stream:
+        # Copy blob_stream into a TemporaryFile so it can be used by magic,
         # which does not accept a file-like object
         shutil.copyfileobj(content, file_stream)
         file_stream.seek(0)
 
-        # Calling .fileno() forces the file to be flushed to disk
-        content_mime_type = m.from_descriptor(file_stream.fileno())
+        content_mime_type = MimeType(magic.from_descriptor(file_stream.fileno()))
     content.seek(0)
 
     if source_filename is not None:
-        source_filename_mime_type = mimetypes.guess_type(source_filename, strict=False)[0]
-        if source_filename_mime_type is not None and source_filename_mime_type != content_mime_type:
-            # Right now, do not rely on `filename_mime_type` for the return value, but
+        source_filename_mime_type = MimeType(
+            mimetypes.guess_type(source_filename, strict=False)[0] or "application/octet-stream"
+        )
+        if source_filename_mime_type != content_mime_type:
+            # Right now, do not rely on `source_filename_mime_type` for the return value, but
             # warn if it's inconsistent with the content.
             logger.warning(
-                'Inconsistent MIME types: content "%s", filename %s "%s"',
+                'Inconsistent MIME types: content is "%s", filename "%s" is "%s"',
                 content_mime_type,
                 source_filename,
                 source_filename_mime_type,