Ok, I see. To maintain perfect quality, you'd use WAV or another uncompressed file format. That's what I'd probably do if I was doing "real" audio editing with a serious tool like Reaper.
I just tried a few files to check and Reaper definitely doesn't like the AAC or M4A extensions. It will read MP3s, but you don't want to convert from one lossy format to another, do editing, and then convert again for distribution. I'd stick with WAV, AIFF, or something similar for the edits.
Brian.