Fix ThumbnailProvider test failure when run from VBox shared folder

While running the thumbnailprovidertest in my VirtualBox shared folder, QFile::rename was giving the error "Text file busy" every time this test runs and causing the thumbnail file to disappear. Saving to the original path after rename fails seems to work.

Alternatively, the function could just save the image directly to the path without using a temporary file.

On second thought, this behavior might be a VirtualBox bug. In case it's relevant: openSUSE 15.4 RC guest, Windows 10 host.

Sorry about making two pull requests for the same thing. I had created an account with the username being my name but I wanted to change it to my alias.