There are mainly 2 kinds of MMS, with or without SMIL. SMIL is Synchronized Multimedia Integration Language defined by W3C. When there is SMIL, the mime-type of body would become multipart-related, if not, it is multipart-mixed.

MMS without SMIL is just a mail with attachments, without any context. On the other hand, MMS with SMIL, then the handset should open the SMIL first and then according defined in the SMIL, it positions images, video or text with defined duration (looks like a slide show).

However, OMA MMS conformance document has limited the use of SMIL and many handset won't support every tags in SMIL.