Hi John,
mp4 and m4a are the same file format, so renaming it as m4a might do the trick.

We chose mp4 as the name because it "sounded better" than m4a (mp4 seems like the obvious succesor to mp4, and I would want mp5 when it comes out). But it looks like Apple renamed mp4 as m4a to distinguish that as an audio file format. I've seen video programs take ownership of mp4 files on Windows too, so it looks like we could be wrong in using mp4. We can always change it maybe an option "Download my files as mp4 or m4a?)

Have Fun!
Peter Gannon
PG Music Inc.