Extensions Property
Specifies one or more file extensions to be associated with the selected file type on a target computer. You must set this property for each file type or a build error will occur.
Note You should avoid common file extensions, as they will overwrite existing file associations on the target computer. For example, using an extension of .htm associates all HTML files with your application rather than with the user's chosen browser.
Remarks
You can specify more than one file extension by separating the extensions with a semicolon. It is not necessary to prefix an extension with a period.