How can I make the KDE file selector dialog recognize *.m4a files as "media"?

In KDE, when browsing for a media file in the file selector dialog, *.m4a files aren't showing up with the Media filter applied:

If I remove the 'Media' filter, the *.m4a file shows up:

How can I get this dialog to recognize *.m4a as a Media file extension?

I verified that *.m4a is registered as an extension for MPEG4 audio in System settings--and as you can see from the second screen shot, KDE does think the *.m4a file is a media file, as represented by the musical note icon. Google mostly points to problems with Amarok, which of course I'm not using here.

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System details:

Debian 7.8

KDE 4.8