Just putting it in an anwswer since it is not accepting my tagged message. I've looked at the topic you suggested, but my issue is that i cant override the xmime:content-type. My request (as the debug in webservices gives it, is like:
But it looks like it is still posted as content-stream: application/octet-stream
Can you attach the WSDL of the webservice?
In an other question this seems to be caused by the wsdl:
(It's about using xmime:base64Binary instead of xs:base64Binary)