Extensions Confusion

I was wondering if someone could tell me how to access my "File Extension" list.I just want to know what it says.A friend told me that there is a program or function that tells me what "File Extensions" I need.

Please have mercy on Kaz,and help.

A friend told me this.

(works if you have Windows... not anything else though)

Go into a folder (any folder), click the "tools" tab and select "Folder Options". When the new window opens, click on the "View" tab and scroll down and uncheck the box that reads, "Hide extensions for known file types".

This will allow you to change te extension of any program or application. Just, make sure you choose a file type that works and remember what it originally was.