MIDI ringingtone download via WAP is based on user download. WML page simply includes a link to a CoD-file, which includes information about the file (name, size, vendor, price, URL to the actual MIDI-file...). After you get the CoD-file to your phone, you can "Receive" or "Reject" the content. If you select "Receive", phone starts to download the content and informs when the download is done.
MIDI-file inside MMS message in just one object in the message. It can be saved to the phone for the later use.
MIDIs are protected content. You can't forward received MIDI-files to anybody.
More info about CoD can be found from http://www.forum.nokia.com/main/1,35452,1_3_1,00.html.
For constructing MMS messages e.g. with MIDI content, you can use Nokia MMS Java Library. It's downloadable from http://www.forum.nokia.com/smsforum/main/1,35452,1_2_7_2_2,00.html.
V / Forum Nokia
RE: 3510: Polyphone Ringtones per SP-MIDI via WAP/MMS