无法通过移动电子邮件打开电子邮件（网络邮件）附件

Question

I sent a mail which contains a table and an attachment. Its working properly in both mobile and PC when I sent the mail to a gmail account, but when I sent the same mail to a web-mail account, I can view the table and download the attachment on a PC (squirrel), but the attachment can't be downloaded from my in-build in-mobile app (e-mail). The attachment is completely visible, and the file size is matching too, but can't open it in mobile. I used the code correctly which is already an posted answer of one question.

My code format is correct as shown below:

$to = "[e-mail]";
$from = "Website <[e-mail]>";
$subject = "Test Attachment Email";

$separator = md5(time());

// carriage return type (we use a PHP end of line constant)
$eol = PHP_EOL;

// attachment name
$filename = "document.pdf";

//$pdfdoc is PDF generated by FPDF
$attachment = chunk_split(base64_encode($pdfdoc));

// main header
$headers  = "From: ".$from.$eol;
$headers .= "MIME-Version: 1.0".$eol; 
$headers .= "Content-Type: multipart/mixed; boundary=\"".$separator."\"";

// no more headers after this, we start the body! //

$body = "--".$separator.$eol;
$body .= "Content-Transfer-Encoding: 7bit".$eol.$eol;
$body .= "This is a MIME encoded message.".$eol;

// message
$body .= "--".$separator.$eol;
$body .= "Content-Type: text/html; charset=\"iso-8859-1\"".$eol;
$body .= "Content-Transfer-Encoding: 8bit".$eol.$eol;
$body .= $message.$eol;

// attachment
$body .= "--".$separator.$eol;
$body .= "Content-Type: application/octet-stream; name=\"".$filename."\"".$eol; 
$body .= "Content-Transfer-Encoding: base64".$eol;
$body .= "Content-Disposition: attachment".$eol.$eol;
$body .= $attachment.$eol;
$body .= "--".$separator."--";

// send message
if (mail($to, $subject, $body, $headers)) {
    echo "mail send ... OK";
} else {
    echo "mail send ... ERROR";
}

Answer 1

There might be a filename missing in the attachment part:

Content-Disposition: attachment; filename=...