Please note that MTOM fails when the receiving application validates your request. This is because Mendix adds unwanted attributes to an XML node in the request, see https://forum.mendix.com/questions/8543/How-to-send-MTOM-attachments-in-a-consumed-web-service.
When the receiving party does not validate the request it should work fine.
We've filed ticket for this issue last week.
I believe they do, also read these release notes.