How to Force Windows Explorer to Always Show You the File Extensions？
This behaviour also causes a lot of problems, particularly to webmasters trying to fix other problems created by Notepad, and so on.
Fortunately, it's easy to fix this problem in Explorer. I'll give the procedure separately for Windows XP, Windows Vista and 7, and Windows 8.
How to Show File Extensions in Windows XP
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".
Displaying the File Extension in Windows Vista and Windows 7
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.
How to Show File Extensions in Windows 8
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.
That's it. You should now be able to see the true extensions of the files in your Explorer windows.