Is it possible to use property expansion in the content of an attachment?

What I want to archive is attating an XML file to a SOAP-Request.

I can't simply include the content of the XML file in the message, because the server interface requiers that the body of the SOAP request is attatched as a file and I can't change the interface.

So my question is, if it is possible to use properties in the attatced file and if yes how it is possible? A similar Question has also been asked here, but it didn't get any answer.