type: Add type for ogg and mp3.
Use the metadata type for the internal type.

Upon loading the file.
Type: find file types from metadata.
License: AGPLv3.

Reason: http://www.gnu.org/licenses/why-affero-gpl.html
File type system as sets.

Which MIME type is which driver is found in a "drivers: MIME → driver" key in
the metadata.