Comment 7 for bug 1384243

That sounds good. The thumbnailer will re-thumbnail if and only if mtime or ctime have changed. So, if you append the timestamp only when there actually was an edit, that will trigger a re-thumbnail only when the time stamp changes.

And setting the image source to the modified URL invalidates the QML cache, so QML triggers a refresh of the thumbnail.

This all sounds good. If you want to verify that it behaves as you expect, just run a trace on dbus.log, and you'll be able to exactly when the thumbnailer does any work for that image.