It's finally out. This addedndum to SwA proposes solutions to the two key problems with SwA:
- How are attachments referenced so that they are included in the message infoset and so that MIME headers do not pose a security threat.
- How does an end point describe MIME content type at the abstract interface regardless of how the actual content is serialized (inline or attachment).

The mechanisms described in this spec are quite simple and, as its name implies, can be implemented as extensions to SwA. I hope to see the industry embrace the concepts in this addendum.