PHP mail: attachement sent but won't open - php

Question

The mail works well and I receive it in my mailbox and I receive the attachment too but I can't opent it, and I'm really in a situation that I can't use any library like SwiftMailer or PHPMailer so I need to make this work.
This is my code:
function mail_attachment($to, $subject, $msg, $from, $file) {
$file_size = filesize($file);
$handle = fopen($file, "r");
$content = fread($handle, $file_size);
fclose($handle);
$content = chunk_split(base64_encode(file_get_contents($file)));
$uid = md5(uniqid(time()));
$eol = PHP_EOL;
// Basic headers
$header = "From: Equipe Grifoflex <".$from.">".$eol;
$header .= "Reply-To: ".$to.$eol;
$header .= "MIME-Version: 1.0\r\n";
$header .= "Content-Type: multipart/mixed; boundary=\"".$uid."\"";
// Put everything else in $message
$message = "--".$uid.$eol;
$message .= "Content-Type: text/html; charset=ISO-8859-1".$eol;
$message .= "Content-Transfer-Encoding: 8bit".$eol.$eol;
$message .= $msg.$eol;
$message .= "--".$uid.$eol;
$message .= "Content-Type: application/pdf; name=\"".$file."\"".$eol;
$message .= "Content-Transfer-Encoding: base64".$eol;
$message .= "Content-Disposition: attachment; filename=\"".$file."\"".$eol;
$message .= $content.$eol;
$message .= "--".$uid."--";
return mail($to, $subject, $message, $header);
}
What am I missing here?