Discarded extension on file download

Hi I am trying to offer downloading files from a web server, using PHP. The files are gzipped text and have all .txt.gz extensions. Now while on Windows systems both IE and Netscape offers to download it under the full name (althought IE adds [some_number] to the base of the file name), under Unix or Linux systems (tested with netscape 4.7) the '.gz' part is discarded from the file name and the file is saved with .txt only. Downloaded file is still gzipped. The part of script serving the files is as follows: header("Content-type: application/octet-stream"); header("Content-Disposition: attachment; filename=$my_filename"); echo($file_body); What am-I doing wrong? Thanks in advance Tom