Aha – so MMS uses SMIL.
“This is a multi-part message in MIME format.
——<200303132002.UAA09403@mmscfe1.mms.mnc033.mcc234.gprs>
Content-type: multipart/related;
boundary=”Boundary_(ID_p5GD6C2ojYJObmQ605bMrg)”; type=”application/smil”;
start=”<941366072>“
“