Roger, I'd guess that the MIDI file is encode as a Multimedia Internet Mail Extension (MIME) attachment. It is in fact part of the complete Simple Mail Transport Protocol (SMTP) message but your email reader (Outlook?) is designed to recognize MIME attachments and treat that portion of the message differently depending on the MIME type. MIDI is easy to deal with because all it has to do is pipe the MIDI information over to your MIDI enabled sound card. Other MIME types may require plug-ins or separate applications to process them properly.

      - Mark