How do I add file types to Windows 8 "File Associations"?

I am trying to associate .scss files with my text editor, but the file extension is not listed under Control Panel > Programs > Default Programs > Set Associations

How can I set this association if the file extension isn't listed? I see no option to add extensions.

Create a text file named example.scss.

1. Double click it.

2. Make sure "Always use as default program" is checked".

3. Select your text editor

I found this question and answer when searching for a solution in Windows 7.

@surfasb's answer didn't work for me as the checkbox Always use the selected program to open this kind of file was greyed out.

@Fred's answer didn't work either, I don't know why but the added file extension didn't show up in Control Panel > Programs > Default Programs > "Associate a file type or protocol with a program"

Instead I had to do the following:

1. Right-click a file with the file extension you want to set default program for
2. Click Properties.
3. In the General tab.
4. On the same line as Opens with: there is a button Change..., press it.
5. Now you get the same pop-up dialog as in @surfasb's answer but with the greyed out Always use the selected program to open this kind of file pre-checked.

Go to Control Panel > Indexing options > Advanced > File types tab.

At the bottom, you should see : Add new extension to list

Type the new extension & click add > OK

Fred