Well, you can just right click the file, select rename and change the
extension. However, if the file is not of the type to which you are
changing it, the rename is superfluous. You can also open the file and save
it as the type you desire by going to the File menu, selecting Save As and
changing the extension accordingly. However, unless you have an application
on your system that supports this file type it likely won't be in the
dropdown list in that box.