This article outlines how to use MTOM.
Validate MTOM: This assertion will ensure that the message is MIME/MULTIPART, with the first part being application/XOP+XML and there being a second piece with a content type of any kind.
Encode MTOM: This will take inline MIME, encode it to base64 and build the MIME/MULTIPART message structure.
Decode MTOM: This will decode the MTOM from the package and place in the payload as inline MIME.
A) When using the Decode MTOM assertion, the message will increase in size by approx. 1/3 in size, which can push message max size constraints. To work around this, modify the value of io.xmlMaxPartBytes.
B) 7.0 will decode MTOM by default for SOAP messages. Modify this behavior by changing the mtom.decodeSecuredMessages value to FALSE.