User creates a file named "foo/bar?.txt" in MacOS (? is an invalid character in windows). Under windows the file will not download and the folder appears to be empty. The user can try to move or delete the folder but the folder re-appears after a few seconds.
Provide the user with a warning that some files could not be downloaded due to incompatable filenames. Provide an automatic resolution, removing the invalid characters. Example: get_valid_filename(line 229) django file utils git