fseeko (data_in, 0L, SEEK_END);
number_to_string (content_length, ftello (data_in));
warc_write_header ("Content-Length", content_length);
fseeko (data_in, 0L, SEEK_END);
number_to_string (content_length, ftello (data_in));
warc_write_header ("Content-Length", content_length);