immagine linkabile dentro mail

[tapu]

per creare un'immagine cliccabile basta fare:
<img src=path immagine />

fin qui tutto ok.

io ora come rendo un'allegato visibile e cliccabile all'interno di una mail

$filez = fopen($f, "r"); 
$contents = fread($filez, filesize($f)); 
$encoded_attach = chunk_split(base64_encode($contents)); 
fclose($filez); 

$msg_body .= "\n\n--Message-Boundary\n"; 
$msg_body .= "Content-type: $filetype; name=\"$titolo\"\n"; 
$msg_body .= "Content-Transfer-Encoding: BASE64\n"; 
$msg_body .= "Content-disposition: attachment; filename=\"$titolo\"\n\n";
$msg_body .= "$encoded_attach\n";
$msg_body .= "$encoded_attach";

            $body_top .= "Content-type: text/html; charset=iso-8859-1\n"; 
            $body_top .= "Content-transfer-encoding: 7BIT\n"; 
            $body_top .= "Content-description: Mail message body\n\n";
    
        //    $body_top .= $msg_body;        
            $body_top .= "\n\n--Message-Boundary\n";    
            $body_top .= "Content-type: text/html; charset=iso-8859-1\n"; 
            $body_top .= "Content-transfer-encoding: 7BIT\n"; 
            $body_top .= "Content-description: Mail message body\n\n";
            $body_top .=  $mess ;
                
            $sql = "SELECT mail_add FROM mail_list_test;";
        $query = mysql_db_query($db_nome, $sql);
        while($row = mysql_fetch_assoc($query)){
            $destinatario = $row['mail_add'];
                $mail_invio = mail($destinatario,$oggetto,$body_top. "Per cancellarsi dalla mail list clicca <a href=\"http://www.link.php/mail_list_delete/cancella_mail_list.php?conferma=si&email=$destinatario\">qui</a></body></html>           ",$intestazioni);
    }