You’re looking for the system variable of “Original File Name”.
In your Output plug-in, right click on the File Name field and you’ll be given a menu. Go to Variables > System. In this final menu fly-out, you’ll see many possible variables to use in your filename, as well as the variables themselves.
Note that any time you see a field name in maroon like this one, you can right click to see a similar menu allowing you to build dynamic inputs.
That’s right. Any output will do as long as it makes a file. They all have the File name option I mentioned and all accept the same variables accessible through the right click menu.