unzip the file contents to the dest_folder
        (create if it doesn't exist)
        and then return the list of files extracted

def extract(self, dest_fldr, password=''):
        """
        unzip the file contents to the dest_folder
        (create if it doesn't exist)
        and then return the list of files extracted
        """
        #print('extracting to ' + dest_fldr)
        if self.type == 'ZIP':
            self._extract_zip(dest_fldr, password)
        elif self.type == 'GZ':
            self._extract_gz(dest_fldr, password)
        elif self.type == 'TAR':
            self._extract_tar(dest_fldr, self.fname)
        else:
            raise('Unknown archive file type')