pgp encrypted message are mostly marked as application/octet-stream

Summary:
We can autodetect if, the content of an mimepart is pgp encrypted.

BUG: 324365
FIXED-IN: 17.12.0

Test Plan: make sure, that attatchment are still handled as attachments and not as "normal" text.

Reviewers: vkrause

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D8554