Okay, took a look on the specs and attached is a memo on how an
implementation of org to MIME could be done.  The MIME delimiters of
SEMI and mml are quite similar so there's already a generic function
that creates representation of a MIME message for both.

I've published this memo on Worg, too, occupying some space in

http://orgmode.org/worg/org-devel.php

The tasks at hand would be: find the functions that attach a file in
mime-edit-mode and mml-mode and look who they can be utilized.

Best
 -- David

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