I experimented with this a bit on my webhost server using Cpanel. I don't believe "audio/mp4" is the correct MIME type. After adding this MIME type, I downloaded an .m4a audio file from my site. Firefox appended .mpg to the end of the file because it considered it a mpeg-4 movie, and then my Windows laptop associated the movie file with QT, of course, rather than with iTunes.
I did some more research and found "audio/x-m4a" to be the correct MIME type. My testing showed this to work correctly. The filename does not get appended with another extension, and the audio file is correctly associated with iTunes. Your mileage may vary.
Here's my source for the info....