There are NO carrier settings to modify in the Settings->About->Network for the MMS like everyone has with the 5.0 carrier file so the setting cannot be changed or modified manually without editing the carrier file itself
+1 ^^^ Right on PezCore:
Actually, it's not a JB, but a carrier update ipcc to the iPhone Carrier Support File. In Settings>Network = AT&T 5.0 while many others w/o MMS are AT&T 4.0......