В HKEY_CLASSES_ROOT\.txt есть еще подключ notepad с подключем shellnew или это только у меня.
Код:

Windows Registry Editor Version 5.00
[HKEY_CLASSES_ROOT\txtfile]
@="Text Document"
"EditFlags"=dword:00010000
[HKEY_CLASSES_ROOT\txtfile\DefaultIcon]
@=hex(2):25,00,53,00,79,00,73,00,74,00,65,00,6d,00,52,00,6f,00,6f,00,74,00,25,\
00,5c,00,73,00,79,00,73,00,74,00,65,00,6d,00,33,00,32,00,5c,00,73,00,68,00,\
65,00,6c,00,6c,00,33,00,32,00,2e,00,64,00,6c,00,6c,00,2c,00,2d,00,31,00,35,\
00,32,00,00,00
[HKEY_CLASSES_ROOT\txtfile\shell]
[HKEY_CLASSES_ROOT\txtfile\shell\open]
[HKEY_CLASSES_ROOT\txtfile\shell\open\command]
@=""
[HKEY_CLASSES_ROOT\txtfile\shell\print]
[HKEY_CLASSES_ROOT\txtfile\shell\print\command]
@=""
[HKEY_CLASSES_ROOT\txtfile\shell\printto]
[HKEY_CLASSES_ROOT\txtfile\shell\printto\command]