MIME-types datastore.
This datastore can handle information from mime.types-style files
and supports basic determination of MIME type from a filename or
URL, and can guess a reasonable extension given a MIME type.
Tuple containing two dictionaries, mapping filename extensions to MIME types: the first dictionary is for the non-standards types and the second one is for the standard types.
Tuple containing two dictionaries, mapping MIME types to a list of filename extensions: the first dictionary is for the non-standards types and the second one is for the standard types.