Show file extensions

Open Windows Explorer. You can do this by clicking "My Computer" from the Start Menu. Note that this is NOT the same thing as Internet Explorer.
Click the "Tools" menu on the menu bar in Explorer the menu bar is at the top of the Explorer window, underneath the window title. Click the "Folder Options" item in the popup menu that appears.
A dialog box appears. Click the "View" tab at the top of the window.
Look for a setting "Hide file extensions for known file types" and uncheck the box beside it to disable it.
Click the button "Apply to All Folders". Then click "Apply" and then "OK".

Click the Start menu. This is the round icon that is on the bottom left corner of your screen.
Type "folder options" without the quotes. Click the line "Folder Options" that appears at the top of the Start menu.
A dialog box with the title "Folder Options" will appear. Click the "View" tab at the top of the window.
Click to uncheck the box for "Hide extensions for known file types".
Click the "OK" button at the bottom of the dialog box.

Invoke the start screen. One way to do this is to move your mouse to the bottom left corner of your screen. In Windows 8, there's no indication that this will cause the start screen to appear, but it will.
Type "folder options" without the quotes. I know that there is no blank field in the start screen to suggest that you can actually type anything. Like many things in Windows 8, arcane knowledge is needed to operate the system.
On the left side of the screen, you will get the disappointing message "No apps match your search". The start screen search facility in Windows 8 can only find something for you if you already know where to find it and can tell it where it is.

Click the line "Settings" on the right side of the screen.
The left side of the screen now shows "Folder Options" as one of the possible results. Click it.
A window with the title "Folder Options" will appear. Click the "View" tab at the top of that dialog box.
Look for the line "Hide extensions of known file types" and click the box beside it to remove the tick.
Click the "OK" button.

Click the icon on the task bar at the bottom of the screen to invoke the Start menu. The icon is the one that looks like a white version of the Windows logo.
Type "folder options" without the quotation marks. There is no blank field on the screen to suggest that you can type anything, but you can. Just type it.
Click the "File Explorer Options" item that appears in the "Best match" list that appears.
A dialog box with the title "File Explorer Options" will appear. Click the "View" tab at the top of the window.
Scroll to find the item "Hide extensions for known file types" and click the box next to it to remove the tick.
Click the "OK" button.

That's it. You should now be able to see the true extensions of the files in your Explorer windows.