Files with <match value="ftypM4B" type="string" offset="4"/> are AAC audiobook files; glob *.m4b. Also files with "ftypM4A" and glob *.m4b, renamed m4a files. The two formats are identical in terms of container, codecs etc. but iTunes allows bookmarks in m4b files, so expected application semantics are different.
Thanks for your bug report! It seems a bit odd that both M4A files with an M4B extension, and files with an arbitrary extension that contain a specific M4B matchlet are both treated as M4B. Is this correct? You say that M4B is a bookmarkable M4A audio file (on Apple systems), so why does it have slightly different contents at all?
OK, this is more complicated than I thought. Apple, through iTMS, sells audiobooks infected with FairPlay DRM (as with their .m4p music offerings); these have M4B magic and .m4b extension. Now, iTunes will only recognise a track as an audiobook if it has an .m4b extension and is AAC in MPEG-4 (or is in Audible infected .aa format, but that's not relevant here). Because of this, providers of uninfected audiobooks (e.g. LibriVox/Gutenberg) offer AAC files with a .m4b extension but M4A magic, so that iTunes users can use them directly as audiobooks (and copy them to iPod as audiobooks): Vendor | Infected | Extn | Magic | Container | Codec | Works in iTunes ---------+--------------+------+-------+-----------+-------+---------------- iTMS | Y (FairPlay) | m4b | M4B | MPEG-4 | AAC | Y Librivox | N | m4b | M4A | MPEG-4 | AAC | Y Note that "bookmarkable" etc. is a bit of a red herring; AFAIK with iTunes/iPod the bookmark isn't stored as a tag, it's stored in the iTunes database and then written out to the iPod database. It's really about what iTunes decides to view the files as.
2007-08-31 Bastien Nocera <hadess@hadess.net> * freedesktop.org.xml.in: Add the audio/x-m4b mime-type, as a sub-class of audio/mp4
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