The W3C XML Protocol Working Group has published three candidate recommendations covering XOP, a MIME multipart envelope format for bundling XML documents with binary data:
Basically this is another whack at the packaging problem: how to wrap up several documents including both XML and non-XML documents and transmit them in a single SOAP request or response. In brief, this proposes uses a MIME envelope to do that. This is all reasonable. I do question the wisdom, however, of pretending this is just another XML document. It's not. The working group wants to ship binary data like images in their native binary form, which is sensible. What I don't like is that the working group wants to take their non-XML, MIME based format and say that it's XML because you could theoretically translate the binary data into Base-64, reshuffle the parts, and come up with something that is an XML document, even though they don't expect anyone to actually do that.
Why is there this irresistible urge throughout the technology community to call everything XML, even when it clearly isn't and clearly shouldn't be? XML is very good for what it is, but it doesn't and shouldn't try to be all things to all people. Binary data is not something XML does well and not something it ever will do well. Render into binary what is binary, and render into XML what is text.