When you open a file, Windows will check the file extension and would launch the application which is set as default for that extension. In Windows XP, you can see the file extension very directly, while in Win7, file extension is invisible as default. If you want to change the file extension or show the file extension, you can follow the following steps:
1. Open a certain file folder. Click “Organize” at the left top.
2. Choose “Folder and search options”
3. There click the “View” Item.
4. In the Advanced settings, uncheck “Hide file extensions for unknown files types” and then click “OK”
5. Now you can see the file extensions and change it easily.