puremagic seems to throw an exception if the file cannot
be detected instead of just "application/octet-stream".

rbtools/utils/mimetypes.py", line 139, in guess_mimetype
    types = puremagic.magic_string(data)
puremagic/main.py", line 341, in magic_string
    info = _identify_all(head, foot, ext)
puremagic/main.py", line 196, in _identify_all
    return _confidence(matches, ext)
puremagic/main.py", line 137, in _confidence
    raise PureError("Could not identify file")