This bundle is indeed a "content transport mechanism" in it's primary intent. There is no expectation that is will be persisted by the Recipient. As stated in the operation definition for the $process-message endpoint: "In addition to processing the message event, a server may choose to retain all or some the resources and make them available on a RESTful interface, but is not required to do so." Thus a content model for persistance such as Document is out of scope.