class mime_mail {
var $parts;
var $to;
var $from;
var $headers;
var $subject;
var $body;

// создаем класс
function mime_mail() {
  $this->parts = array();
  $this->to =  "";
  $this->from =  "";
  $this->subject =  "";
  $this->body =  "";
  $this->headers =  "";
}

// как раз сама функция добавления файлов в мыло  
function add_attachment($message, $name = "", $ctype = "application/octet-stream") {
  $this->parts [] = array (
   "ctype" => $ctype,
   "message" => $message,
   "encode" => $encode,
   "name" => $name
  );
}

// Построение сообщения (multipart)
function build_message($part) {
  $message = $part["message"];
  $message = chunk_split(base64_encode($message));
  $encoding = "base64";
  return "Content-Type: ".$part["ctype"].($part["name"]? "; name = \"".$part["name"]."\"" : "")."\nContent-Transfer-Encoding: $encoding\n\n$message\n";
}

function build_multipart() {
  $boundary = "b".md5(uniqid(time()));
  $multipart = "Content-Type: multipart/mixed; boundary = $boundary\n\nThis is a MIME encoded message.\n\n--$boundary";
  for($i = sizeof($this->parts)-1; $i>=0; $i--) $multipart .= "\n".$this->build_message($this->parts[$i]). "--$boundary";
  return $multipart.=  "--\n";
}

// Посылка сообщения, последняя вызываемая функция класса
function send() {
  $mime = "";
  if (!empty($this->from)) $mime .= "From: ".$this->from. "\n";
  if (!empty($this->headers)) $mime .= $this->headers. "\n";
  if (!empty($this->body)) $this->add_attachment($this->body, "", "text/plain");    
  $mime .= "MIME-Version: 1.0\n".$this->build_multipart();
  mail($this->to, $this->subject, "", $mime);
}
}

Вот и все, привожу пример использования класа:

$attachment = fread(fopen("file.zip", "r"), filesize("file.zip"));  
$mail = new mime_mail();
$mail->from = "my@e-mail.com";
$mail->headers = "Errors-To: [EMAIL=my@e-mail.com]my@e-mail.com[/EMAIL]";
$mail->to = "user@e-mail.com";
$mail->subject = "PHP atachment";
$mail->body = "Get your file!";
$mail->add_attachment("$attachment", "file.zip", "Content-Transfer-Encoding: base64 /9j/4AAQSkZJRgABAgEASABIAAD/7QT+UGhvdG9zaG");
$mail->send();