mp4 is missing from the mimetypes.py list of valid mimetypes. mp4 is 
registered with IANA and is defined in the mpeg-4 standard as a container 
for mpeg-4 codecs such as h264. Lack of this definition means the format 
cannot be recognised by software dependsing on the mimetypes modules, that 
don't use one of the suggested local mime definitions (OpenBSD).