from datetime import datetime


def generate_filename(prefix,
                      filetype='tar',
                      fmt='%Y%m%d-%H:%M:%S:%f'):
        """
        Generates a string to be used as the bundle filename.
        Format: <prefix>-<data-fmt>.<filetype>>
        """
        now = datetime.strftime(datetime.now(), fmt)
        return '{0}.{1}'.format('-'.join([prefix, now]), filetype)