def build_filenames(name_list, ext_list):
    """
    Build a list of filnames using the name list and extention list.
    """
    if len(ext_list) == 0:
        names = set(name_list)
    else:
        # Force our extension to start with a dot (.)
        ext_list = ['.{0}'.format(ext.lstrip('.')) for ext in ext_list]
        names = set(['{0}{1}'.format(n, e) for n in name_list for e in ext_list])

    print('[+] Built {0} filenames.'.format(len(names)))
    return names