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