[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