change standardprogram to open a file to "unknown"

Hi there !

First of all: I'm not to good in english - but I try my very best.

This is my problem:
I have changed the standardprogram to open .Ifo - files from "unknown" to
"adobe acrobat" by mistake. And now I want to change that back again.

Is there any possibility to do so? Or can I just select another program?

Hope, somebody can help.

Thanks a lot !