the OS does not support wav files. i believe 3rd party applications that have audio in them calls to the API that supports various audio file types (ADPCM, MIDI, etc) for the 7100 rather than creating new file type possibilities via their own API (if that would even be possible). i personally do not see there being a limitation of the hardware besides the software.
but i'm curious... how large are these wav file attachments? i would think they would be rather large, but i'm not really sure. we have something at work that does this for our Tech Support group's ticket system, but i've never bothered looking at the file size associated with the wav files. i don't really see the benefit of being able to play extremely large wav files via email attachment.