I've found two examples on https://lore.kernel.org/lkml/
where the messages declared themselves to be "multipart/mixed"
but were actually plain text:
<87llgalspt.fsf@free.fr>
<
200308111450.h7BEoOu20077@mail.osdl.org>
With the mboxrd downloaded, mutt is able to view them without
difficulty.
Note: this change would require reindexing of Xapian to pick up
the changes. But it's only two ancient messages, the first was
resent by the original sender and the second is too old to be
relevant.