Err Msg: The Extension '.<ext>' Is Already In Use By File Type '<Name>'

When you try to add a file type to the Registered File Types list, you receive the following error message, even if you cannot find the file extension in the list:

Multiple file types can be registered to one application. In this case, the file type does not appear in the Registered File Types list, but instead appears in the details for another registered file type.

For example, the registered file type Microsoft Word 6.0 Document can contain .RTF, .DOC, and .DOT extensions. Even though these file types do not appear in the Registered File Types list, they are registered to Microsoft Word 6.0.

To change the association of a file type that does not appear in the Registered File Types list, follow these steps:

1. In My Computer or Windows Explorer, click a file of the appropriate type, and then press and hold down the SHIFT key while you right-click the file.
2. On the menu that appears, click Open With.
3. If you are prompted to, enter a description for the file type. In the "Choose the program you want to use" box, click the program you want to use for this file type.
4. If the "Always use this program to open this file" check box is not selected, click the check box to select it.
5. Click the OK button.