Audible AAX files are standard ISO Base Media File Format (ISOBMFF) files,
often called "MP4" or "Quicktime". Metadata can thus be extracted with
the taglib "MP4" code path.
The test files have been created with ffmpeg and then edited with okteta/
exiftool to resemble real AAX files, which can not be included for
copyright reasons.
https://gitlab.freedesktop.org/xdg/shared-mime-info/-/merge_requests/76
is required to correctly detect AAX files.