You can Tweak windows to show the file extensions for every single file you have on your computer. So, for example you have a bunch of JPEG pictures, one named "picture". Without the showing of file extensions enabled, it will show as just "picture", but with it enabled it will show as "picture.jpg". This is useful for a number of reasons, for example picking out the desired filetype quickly, and makes life easier for web developers and computer programmers to gain access to their desired files with the right extensions.

This is how you do it:

1. Go to any folder (e.g. My documents) and click tools at the top of the screen.
2. Click on "Folder options"
3. Click the "View" Tab on the popup window
4. In the box with a list of options in it, scroll down to "Hide extensions for known file types" and uncheck it

Before:


After: