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