So, I suggested:
content-type: application/x-zip-compressed; encapsulates=application/pdf
sorry, this is just unacceptable. there are too many things (e.g. mail
filters) that expect the content-type to actually describe the content.
And even if you get past those expectations, the multipart/signed experience
shows that any expectation that a implementation-defined handler for a type
isn't likely to be able to turn around and feed additional parts back into
the agent.
We have a well-defined structure. We need to stick to it.
Ned