Well, more specific information about what you are doing would help. But...
Yes, the media type of MMS content is defined in the "Content-type" header of each media object in the MMS message body. Preceeding the data of each media object, there will be some component headers which might look like this:
The "Nokia Phone Messaging Characteristics" document lists the supported MMS content types for each Nokia phone.
It can be found at www.Forum.Nokia.com under "Technologies" >> "Messaging" >> "Multimedia Messaging" >> "Documents"