Excerpts from Ruthard Baudach's message of 2013-07-05 12:32:48 +0200:
> sup displays these emails as empty mails with an html attachment.
> 
> It would be great if sup was able to distinguish between actual html
> attachments and such deficient mails, and would extract and display
> the text part of these mails.

This can be solved with mime-decode hook [1].

[1] https://github.com/sup-heliotrope/sup/wiki/Viewing-Attachments