In Explorer, go to Tools | Folder Options | File Types, find the extension you're looking for, and click Delete.
Or, if Delete is grayed out, click Restore (to set it back to the defaults), then Delete.
In Explorer, go to Tools | Folder Options | File Types, find the extension you're looking for, and click Delete.
Or, if Delete is grayed out, click Restore (to set it back to the defaults), then Delete.