judging by that message the extension is being seen as ". mp3" and not ".mp3" (note the extra space), please check that you have not got an extra space by mistake. AFAIR there was also a problem with this area once, what version of dotnetnuke are you running, perhaps it's a version where the additional types were not correctly respected.

Cathal

One other item to consider, since you mentioned some files are very large, is this entry in the web.config: maxRequestLength="8192"

That number limits the size of uploads.  Although I've had this problem in the past, I don't recall the error message.  The number represents the number of kilobytes so 8192 is 8 meg.  It needs to be changed in two places in the same line of the web.config.

Mike