SOAP attachments

I want to use SOAP attachments with the mozilla SOAP API to sent and
receive large mime.
I found out that the c++ source in firefox has place holders for that
things but it's not yet implemented.
Does anybody know how to work around this issue?
May it be possible to get it work with a custom en-/decoder? (or is
that only for the XML payload)
If so has anybody a piece of sample code which implements that?
I think the worst way (from my little networker point of view) would be
to create a whole new SOAP javascript "library" (even if that would be
my last resort :-)