[Comic Thumbnailer] fix CBR thumbnail generation

Summary: The .desktop file was using an incorrect mimetype.

Reviewers: broulik, #frameworks, elvisangelaccio

Reviewed By: elvisangelaccio

Subscribers: elvisangelaccio, kde-frameworks-devel, kfm-devel

Tags: #dolphin, #frameworks

Differential Revision: https://phabricator.kde.org/D15367