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 (closed).