Issue: Permit "message" attribute in mime:content binding

The <mime:content> is defined with a "part" and a "type" attribute spec.

E.g.                         <output>
                                    ..
                                    <mime:part>
                                         <mime:content part="docs"
type="text/html"/>
                                    </mime:part>
                                </output>

I think it would be very useful to permit the part to come from a
different message just as it is done with the <soap:header > binding.
Like <mime:content message = "tns:rnheaders" part="serviceHdr"
type="text/xml"/>

Regards, Prasad