...We need to somehow
differentiate between the multipart which comes from in-stream parts (a
hebrew sentence in the middle of an otherwise english paragraph) and the
multipart which comes from the attachment model...
Rather than calling the latter multipart/related, consider an alternative:
keep multipart/mixed for the latter, and use (say) multipart/sequential
(by analogy to multipart/parallel) for the former.
Henry Spencer at U of Toronto Zoology