Open Unknown Files With Notepad

Add Notepad to the contextual menu (right-button menu):

Create [HKEY_CLASSES_ROOT\*\shell\Open with Notepad\command], click on command, then double-click on "(Default)" in the right pane and edit the string to read notepad.exe %1

Open unknown files in Notepad when clicking on them:

- Create [HKEY_CLASSES_ROOT\Unknown\shell\open\command], click oncommand, then double-click on "(Default)" in the right pane and edit the string to read notepad.exe %1

- If Notepad still does not open unknown files when you click on them, delete the key openas (export it first)

Note: In Windows Explorer, Open With > Choose Program... will not work anymore.