Hi Mike,
I'm not sure, but I think you may have misunderstood the idea behind
"enclosures". The enclosure is just a link to your original media file
but it allows feedburner to record statistics about the use of the
media. Changing the type of media will only change what your feed
states as the media type but at the end of the day the media will
always be whatever the media is regardless of what your feeds xml
states.
This is how I understand the "enclosures", but I'm a relative newbie
myself so other users please feel free to jump in and comment.
You may have already seen this but I found the following page quite
helpful:
http://www.google.com/support/feedburner/bin/answer.py?answer=79581&topic=13075
Apologies if I've misunderstood your question, hope this helps.