Set content disposition of the download.
"Content-Disposition" is not HTTP compliant, but most browsers follow this header, so it was borrowed from MIME standard. It looks like this: "Content-Disposition: attachment; filename=example.tgz".
string [$disposition = HTTP_DOWNLOAD_ATTACHMENT] - the disposition of the download (either 'attachment' or 'inline')
string [$file_name = null] - the file name the browser's download window should show