Actually, since MMS is an old thing since 2G. I should say, better to refer to OMA stardard for the latest information. (Multimedia Messaging Service
Encapsulation Specification)

Same as every protocol, MMS PDU includes headers and body. The headers includes some user addresses in form of phone numbers (MSISDN) or Email addresses. There are also something about priority, delivery report, creation date... The body itself is actually a MIME Body.

Then the whole MMS body will be encoded according to that Encapsulation Spec. Inside the whole MMS body, the difficult part to understand is the MIME-TYPE, which could end up to be very long with many parameters.