Ok, que tal usar o Associate? você pode baixá-lo aqui: link é pequeno e portátil.
Associate is a command-line utility for assigning file associations.
This will allow you to double-click a file and have it
load in the appropriate program.
It also includes the ability to set a file type icon and startup parameters,
and it automatically refreshes Explorer. Type "associate -?" for details.
Syntax: associate [extension] [program]
Example: associate .txt %WINDIR%\notepad.exe