Windows delete file type association

Some extensions in Windows are automatically registered and assigned to be opened by a specific application, others can be manually set via the Open with... dialog or Control panel->Default programs. In either case however, you cannot delete an association.

You may do so by directly editing the registry (Start->Run->regedit):

Assuming you'd like to remove the association for the EXT extension, browse to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\

And delete the subtree named .EXT (with the dot before). Now browse to HKEY_CLASSES_ROOT\ and delete the subtree named .EXT (again, with the dot before) and EXT_auto_file