Hides .p7m attachments
When a multipart/mixed message contains an application/octet-stream attachment whose content can be interpreted as a PKCS#7 Message, then Evolution interprets it as a signed message, which is wrong, because that .p7m attachment is not relevant to the other parts in this multipart/mixed group.
This had been reported on the mailing list at:
https://mail.gnome.org/archives/evolution-list/2019-June/msg00117.html
and continues here:
https://mail.gnome.org/archives/evolution-list/2019-July/msg00001.html