By default when double clicking a file with an unknown filetype, then it starts with "Open with..". It is possible to change the default behavior to instead open the Notepad. One can still access the "Open with..." option by holding down the Shift-key and right-clicking the file.

REGEDIT4

[HKEY_CLASSES_ROOT\Unknown\shell\open]
@=""

[HKEY_CLASSES_ROOT\Unknown\shell\open\command]
@="NOTEPAD.EXE \"%1\""

Note instead of modifying the registry to make notepad default viewer for unknown file types, then one could use the Send To-folder for easy access to the favorite file-viewers.

Related Configure file types to include your own useful actions
Related Extend context menu with "Open with" for all files