First,u need to enable viewing of extension on your PC

1)Double click on 'My Computer' icon your desktop.

2)From the menu on the top ,click Tools And Select the folder options.

3)In folder options,go to the 2nd tab on top called View.

4)There is a box that says "Hide Extension"for known file types.

5)Uncheck this box,Click 'Apply' and 'ok'.

6)Now every file have an extension telling you what it is.