Specifies a default file extension.
property DefaultExt: string;
DefaultExt specifies a file extension that is appended automatically to the selected file name, unless the selected file name already includes a registered extension. If the user selects a file name with an extension that is unregistered, DefaultExt is appended to the unregistered extension.
Extensions longer than three characters are not supported. Do not include the period (.) that divides the file name and its extension.