Web Service Call - Content-Type is overwritten

Hi,
I am trying to call a SOAP web service that expects a binary file as one of the parameters. I am using an ExportMapping to create the SOAP Request Body.
The web service that I am calling will only accept the following content type, which I have specified under custom HTTP headers

Content-Type: text/xml; charset=utf-8

 However when the SOAP call is made the content type is 

Content-Type: multipart/related;start="<rootpart*23149b8d-9269-4cbc-b9ab-c7b2f16823c3@example.jaxws.sun.com>";type="application/xop+xml";boundary="uuid:23149b8d-9269-4cbc-b9ab-c7b2f16823c3";start-info="text/xml"

Any ideas how to change this?

Thanks you