manually set format in jpeg thumbnail creator
Summary:
Otherwise the QImageReader sets the format based on the file name suffix, which might be misleading, e.g. a jpeg file named image.png.
CCBUG: 220496
Reviewers: dfaure, elvisangelaccio, Frameworks
Reviewed By: dfaure, elvisangelaccio
Subscribers: kde-frameworks-devel, kfm-devel, ngraham
Tags: Dolphin, Frameworks
Differential Revision: https://phabricator.kde.org/D13316