i tried to open an mp3 file that was emailed to me. and to my surprise got an error message- unsupported format. after about 2 hrs on the phone with RIM i learned that the device can handle mp3 files but the BB server isnt set up for that! they suggested that we request it in the next update.
If this is important to you as it is to me please email rim at:
and aske them to make the two compatible so you can open mp3 attachments.