How can I change the program associates to open an attachments?

Answer 1

There are two ways, another person said one way, here's the other:
Find an example of the file you want to change, right click on it, choose "open with..." find the program on the list, click "always use this program"...
There you go.

Answer 2

Windows Explorer - Tools - File Options - Highlight the type of program you want to change - Change

Note this will make that program (that you change it to) open all programs with that extension name in the future.