Adding custom extensions to the list has no effect. You can only use it to remove extensions and make Mp3tag ignore these files then.
I don't know if it is save to edit m4p files in Mp3tag (a first test from me says yes), if you want to try it you should work with copies first. Rename them to m4a
And then please test if iTunes still plays the files. If it works, maybe m4p can become a supported extension.
Thanks for the clarification. If you remove a supported extension temporarily, can you then add it back so that mp3tag will again recognize that filetype?
Testing done and it appears successful to me too!
Copied an m4p (iTunes store, protected aac) file.
Changed the copy to an m4a extension
Loaded the copy into mp3tag, appeared to load fine.
Edited tags and saved changes to the copied file.
Added the copied file to the itunes library (still with m4a extension). Modified tags showed up as expected. File played fine.
Deleted file from itunes library and changed extension back to m4p.
Added the file back to the library - everything still seemed to behave as expected.
I tried a couple of other files, all with the same positive results.
At least from this user's perspective, it would seem that m4p files could be supported.
As far as I know, the m4p files are just regular m4a files (aac encoding) that have a few additional quicktime atoms to handle the FairPlay DRM. I don't know if adding/removing tags would change the atom order, and even if it did -- I don't know if that order matters....