Split Audible audio mime types

The old "Format 2/3/4" audible files are proprietary formats, while the
newer "Enhanced" type is actually an ISO Base Media File Format compliant
file (aka ISOBMFF/M4B/Quicktime). The audio stream is encrypted, but
meta data (author, title, cover image) are unencrypted and can be used
by standard tools.

Fixes #134.
1 job for issue_134_split_audible_mimetypes in 2 minutes and 18 seconds (queued for 4 seconds)