Isn't m4b an audiobook?
If this file has chapters then it is not supported by MP3tag, see this German thread (if you like)
/t/7318/1
or this one:
/t/10644/1
which at least has some English sections in it.
As it is not generally impossible to edit m4b files (they just should not contain chapters), they are still included in MP3tag.
You can modify the list of treated extensions in File>Options>Tags.
The D&D mechanism does not analyze a file and its contents, just the extension. So it would not be possible to find out if the files are legal ones during D&D.