De acordo com emacswiki :
Create File Associations
Use Windows command-line commands ‘ftype’ and ‘assoc’:
ftype txtfile=emacsclientw -na runemacs "%1" ftype EmacsLisp=emacsclientw -na runemacs "%1" ftype CodeFile=emacsclientw -na runemacs "%1" assoc .txt=txtfile assoc .text=txtfile assoc .log=txtfile assoc .el=EmacsLisp assoc .c=CodeFile assoc .h=CodeFile