To the best of my knowledge, the SoapUI-generated request you posted is a correct multipart request – nothing seems to be stripped from it. I also believe that this part of SoapUI is well proven, so I'd be surprised if our tool proved to be the culprit in this case. :-)
Unfortunately Arian got the versions of MVC mixed up in an earlier response. As a matter of fact, it is early versions of MVC 4 (not MVC 3) that have had problems processing certain multipart requests. Can you check what version of the framework you're using?
Meanwhile, we will try to reproduce your issue on another platform (i.e. not MVC 4).
Kind regards,
Manne
