Week or so ago this site got some hits from a post at Reddit which served to remind me that "Uh, this site's been around for thirteen years." One of the comments reported that "I used a file renamer script that renames files based on the tracks metadata. It changed all my .m4a tracks to .mp3 and broke a lot of my library even though I told it to not replace file extensions." Sorry about that.
The original version of File Renamer (which was written by a fellow named Tino Heth) was posted in January 2002 and it hasn't been updated since. Back then, there were only MP3s, so the script didn't have to account for M4A. Unfortunately, it didn't account for any other extensions either. So I thought if people are still talking about it after all these years I'd better update it.
You can download it here.