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Step 1
Go to the toolbar of any Windows Explorer window and select the "Tools" menu item. Select "Folder Options …" You also can bring up the "Folder Options" screen by opening the "Folder Options" icon from the Control Panel.
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Step 2
Select the "File Types" tab from the "Folder Options" screen and scroll through the list in alphabetical order to see if the desired extension already exists.
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Step 3
Create a new file extension by clicking on the "New" button to bring up the "Create New Extension" dialog box. Enter the extension in the "File Extension" box. Select the "OK" button.
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Step 4
Scroll through the registered file types and pick the one you wish to associate. Click on the "Change" button to bring up the "Open With Dialog" box. Select the file you wish to associate with your extension and click on the "OK" button. Click the "Close" button on the "Folder Options" screen to close it.
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Step 5
Double-click on any file with the extension you just associated. Windows should open the file with the program you selected in the previous step. Many applications will associate file extensions with the appropriate programs when that application is installed.
