How do I change "open file with" to default file type assist

I have a game called TF2, and I made some replays.
Well, the replays apparently are a file type that my players did not support, and I went to "open file with" and picked Windows Media Player.
When they didn't play, I wanted to change it back to the original unknown type file.

When I originally clicked on the file, it opened up the default Windows "File Type Assistance" box, but now I cannot find it, nor can I change it back. I don't want to have to keep closing Windows Media Player each time I see those files. Plus, I want to use the "File Type Assistance" program to actually find file types and openers.