Blame Micro$oft for their default settings.
In the explorer, goto 'Tools', 'Folder Options', 'View' tab, uncheck the 'Hide exptensions for known file types'
Then hit 'OK' and find that file and rename it back.
For further edits, you don't have to rename the file. Just open Notepad, and drag the file onto Notepad to open it there. (That is if you want to use Notepad anyhow)