How to Fix a File Extension
On computers, the three letters after the period in the file name are known as the "file extension." The file extension explains to the operating system what type of data the file contains, and the operating system uses that information to select the application used to open the file when it is double clicked. If the extension of a file is incorrect, it may not open in the correct application. Correct the file extension to make the file open in the application of your choice when you double click it.
Click "Start," then click "My Computer" if you are running Windows XP or "Computer" in Windows Vista or 7.
Click "Tools," then click "Folder Options" in Windows XP. In Windows Vista and 7, click "Organize" then click "Folder and Search Options."
Click the "View" tab. Remove the check from the box labeled "Hide extensions for known file types" and click "OK." Extensions for all files will now be shown.
Right click on the file that you need to change the extension of, and click "Rename." Delete the current extension, and type the correct one. If the file has no extension, put a period after the file name and enter the extension. Hit "Enter" when you are done, and double click the file to open it.
Tips & Warnings
If you do not know the correct extension for a file but are sure that it contains valid data, feel free to guess. Changing the extension of a file does not change the data that it contains, and if the file does not open correctly, you can simply change the extension again. A list of common file extensions appears in the "Resources" section of this article.