Why can't you see the file extensions?
Most of the time seeing a file's extension isn't crucial, and then there's times when you wish you could tell an .avi from a .jpeg, or perhaps hunt down that pesky piece of a virus that just hussled past your anti-virus software and boogied into the Windows directory...
So why can't you see the file extensions? It's because they're hidden, and here's how to unhide them...
Windows XP
Open Windows Explorer (not IE) by clicking Start, Control Panel or My Computer, or anyway you like.
Now click on Tools and then choose Folder Options from the menu
Choose the View tab and scroll down a bit. Uncheck the box next to "Hide extensions for known types"
Click Ok and you're done.
Windows 7 method
In Explorer view (not IE, but Windows Explorer, or 'Computer')
On the Folder Options dialog, select the View tab.
Then deselect the option "Hide extensions for known file types".
Click Ok and you're done...
