def METHOD_NAME(self):
    """
    Run the dialog and get the filename.
    If this is a save dialog and the file name is missing the extension, append the file extension.
    If the file name with the extension already exists, show a overwrite dialog.
    If this is an open dialog, return a list of filenames.
    Returns:
        the complete file path
    """
    filenames = Gtk.FileChooserDialog.get_filenames(self)
    for filename in filenames:
        if not path.exists(filename):
            self.show_missing_message(filename)
            return None  # rerun
    return filenames