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:
- Right-click a file with the file extension you want to set default program for
- In the
- On the same line as
Opens with: there is a button
Change..., press it.
- 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.