PHP код:
$filename = "attached.txt";
$fd = fopen($filename, "r");
$data = fread($fd, filesize($filename));
fclose($fd);
$boundary = "b" . md5(uniqid(time()));
$header = "Content-Type: multipart/mixed; boundary = $boundary\n\n";
$data = chunk_split(base64_encode($data));
$multipart = "$boundary\n\n";
$multipart .= "Content-Type: application/octet-stream; name=\"$filename\"\n";
$multipart .= "Content-Transfer-Encoding: base64\n";
$multipart .= "Content-Disposition: attachment; filename=\"$filename\"\n\n";
$multipart .= "$data\n\n$boundary\n\n";
$message = $header . "\nTest\n" . $multipart;
В $message правильно все постороил?