Signed messages not being rendered fully

Here is an example: https://www.nntp.perl.org/group/perl.perl6.compiler/2017/09/msg16767.html

> I’m guessing it needs to support whatever “attachments” the mail is being sent as. Patches welcome!
>
> – Ask

Unfortunately I won't be able to work on it any time soon, but here's a ticket so that it doesn't fall through the cracks.