The File Type Group isn't what matters here. The viewer doesn't make choices based on that.
If you want to continue using the viewer for other file formats, and only change PSD, then the easiest thing to do is:
- Settings > File Types.
- Find the PSD type and double-click it.
- On the Events tab, edit the Left double-click event.
- Make it run
FileType ACTION=shellex
That will make Photoshop (or whichever application is associated with PSD files) open the files on double-click, while the other image types still go to the viewer on double-click.