Probe attachment mimetypes ourselves when there's doubt

Summary:
When encountering attachments without a proper mimetype, let's see if we
can find something better. This helps for example with airline booking
confirmation emails with iCal attachments, many have those but all samples
I have mark them as application/octet-stream, making them inaccessible for
KMail.

Now we give the corresponding plugins a chance to process these cases too.

Reviewers: knauss

Reviewed By: knauss

Subscribers: knauss, #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D8551