By default, when you have this app installed, you can find it here:
Applications > System Tools > Configuration Editor
Go to:
desktop/gnome/thumbnailers
There you can find every file type that has preview enabled. To disable all of them at once just check "disable_all" in the "thumbnailers" entry, or you can just find the media type you don't want to have preview generated (they are listed as a sub-directories of "thumbnailers") and uncheck "enable" key.
