Chapters do not show for m4a files

[flaurafauna]

Chapters do not show up for files with m4a extension even if they contain chapter data.

As an example, here is a book stored as an m4a:

And here is the same book if the extension is simply renamed to m4b:

[rdbende]

Hi! This issue was actually fixed not that long ago, but I haven't got around to release a new version yet.