According to the article "Fear of Attachments" by Steve Loughran (damn good title ) it is possible to send the filename with MIME inside a header, but not so with DIME. The code sample in the article passes the filenames as a separate parameter.
I'm not sure but it seems that .NET only supports DIME out of the box. I don't want to lock out .NET users. There are third party libraries though.
However there is the real possibility the each platform will serialize identical attachments in different ways MTOM between Java and .NET - theoretically the other side should be able to handle it ... theoretically.