Allow uploading of GIFs for users.

Discord Nitro accounts can upload GIFs as avatars, so the image
mimetype checker code should check for the GIF magic number too.

Signed-off-by: SunDwarf <[email protected]>

discord/utils.py

@@ -246,6 +246,8 @@ def _get_mime_type_for_image(data):
         return 'image/png'
     elif data.startswith(b'\xFF\xD8') and data.endswith(b'\xFF\xD9'):
         return 'image/jpeg'
+    elif data.startswith(b'\x47\x49\x46\x38\x37\x61') or data.startswith(b'\x47\x49\x46\x38\x39\x61'):
+        return 'image/gif'
     else:
         raise InvalidArgument('Unsupported image type given')