Comment 3 for bug 40874

Thanks for your bug. That's known upstream: http://bugzilla.gnome.org/show_bug.cgi?id=324656

The preference doesn't apply to video because it doesn't require to download the video to snapshot a frame

If you want to stop using the feature for a mimetype you can use gconf-editor and modify the "/desktop/gnome/thumbnailers/<mimetype>/enable" gconf key