Legacy and new Audible file formats should have distinct 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.
The old formats use the .aa extension, while the ISOBMFF ones use .aax.