Let's start with Windows 7. Of course, the most simple and effective way for all operating systems is a way to rename a file in any file manager. For example, Total Commander. But if you do not, the problem is still manageable. Consistently perform the following steps.
Open any file manager window of Windows 7, press the Alt.
Output: should appear menu: File, Edit, Tools, etc.
Select "Tools", it "Folder Options."
In the popup window, select the "View" tab.
Remove the check mark from the line "Hide extensions for known file types."
Now we have Windows XP. Again, in order.
Go to the "Start" menu.
Next: "Settings" and "Control Panel."
In the "Control Panel" open "Folder Options."
In the opened tab "Folder Options", uncheck the line "Hide extensions for known file types", thus do not forget to press the "Apply" and "OK."
Now you can see the file name extension. For example, .doc.
Right-click, select from menu "Rename".
Rename the file by specifying your desired extension.
Most have to rename images and photos, but everyone just take off. Enough, for example, to Total Commander to stand on the desired file and click the right mouse button
In the menu that appears, select "Rename" and instead, for example, ".png" (Portable Network Graphics) prescribe ".jpg"