Add warning in case an invalid mimetype is requested here.

Summary:
This is never supposed to happen, but a bug of mine in QMimeDatabase
alias resolving, in qtbase dev, actually led to this.

Test Plan: the warning now appears for text/directory with qtbase dev. Yes I'll fix that Qt bug ;)

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D10888