To associate a file name extension with a file type
Open Folder Options in Control Panel.
Click Start, and then click Control Panel.
Click Appearance and Themes, and then click Folder Options.
On the File Types tab, click New.
Type a new or existing file name extension, and then click Advanced.
In Associated File Type,
Type or select New to create a file type to associate with the file name extension.
Type or select a different file type to associate with the extension.
When you type a file extension in the Create New Extension dialog box, the Associated File Type list displays the file type that is associated with that extension. To select New, scroll to the top of the list.
For information about how to make modifications to the file type, click Related Topics.
To change the program that opens a file, on the File Types tab, under Registered file types, click the file type, and then click Change.