Creating a new file extension and associate an application

Hi,
I want to create a new file extension named ".emsof" and associate any text editor with it like notepad++ in windows 8.1
Moreover all of these files should be searched by having filter of "*.emsof" like any other ordinary files.
Please guide me about how to do it?
Thanks.

You can just create a file with that extension and on a right click menu go to "Open With" and associate with a program you want. If you want it to stay like that, choose "Choose Default program".
Alternatively, you can open it's file properties and set default there. BTW, you can create a "Dummy" (empty) file with that extension. There's other ways but this ones are simplest.