This is somehow the opposite of #7493. When a thumbnailing task is stuck (for example because there's an issue in the file or in the thumbnailing app) it should be possible to cancel the task and kill the thumbnailing app to prevent the system to go havoc. This was a Debian feature request against thunar-thumbnailers (http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=601435) but I file it in tumbler and I'm not even sure it shouldn't be in the calling application (so Thunar or ristretto or ...).