Summary:
Same solution (for finding the format string for QImage from the
mimetype) as the one used by Spectacle.

Test Plan:
Saving still works as PNG, JPG (to test multiple extensions),
and WEBP (to test a 4-letter extension).

Reviewers: broulik, andreagenor, tcanabrava, vkrause

Reviewed By: vkrause

Differential Revision: https://phabricator.kde.org/D25600