Otherwise PreviewJob will refuse to re-use cache files generated by the directory thumbnailer.
Noticed that when I had a folder with a PDF in it (which takes a while to generate) that despite the folder thumbnails having generated a cache file, it would not use it when I entered the directory and instead recreate it again.
With this patch the thumbnails generated by the folder thumbnailer are accepted and previews when entering a folder with few files for the first time is noticeably faster. It doesn't change perception a lot when there are many files.
I don't have access to the plugin there so I don't add the version information to it. Not sure this could be a problem? It's a bit annoying that thumbnailer KIO duplicates so much code from the preview job already :/