Fix runtime warning about application/x-pkcs7-signature.
Summary:
The kmime code looks for it as fallback, but the registered mimetype is
application/pkcs7-signature without the "x-".
Test Plan: Allen will test :)
Reviewers: winterz, dvratil
Reviewed By: dvratil
Subscribers: dvratil, KDE PIM
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D5859