Image files with wrong, but known, file name extension fail to load

If a valid jpeg file, say, is renamed to "foo.png", then QImage / QImageReader will by default report the file is invalid and fail to load the image. On the other hand, if it is renamed "foo.xyz", it will be recognized as jpeg (on content check) and load fine.

This is inconsistent, and the former situation is probably more likely anyway, and so should be handled by default.