The W3C XML Protocol Working Group has published the last call working draft
Assigning Media Types to Binary Data in XML. This spec attempts to preserve the original MIME media type of Base-64 encoded binary data stuffed in an XML element.
The mechanism by which this happens
is an xmlmime:contentType attribute for indicating
the media type of XML element content whose type is xs:base64Binary. It also defines an expectedMediaType for use in schema annotations
to indicate what the contentType attribute may say.