$to  = "<mail@gmail.com>" ;     
$subject = "Тема листа"; 

$message ="Текст листа"; 
$filename = "$file_name.csv";
$filepath = "./".$filename;

    $boundary = md5(uniqid(time())); 
     
    $mailheaders .= "From: Від кого <info@mail.com>\r\n"; 
    $mailheaders .= "Reply-To: info1@mail.com\r\n";    
    $mailheaders .= "Subject: Hello\r\n";  
    $mailheaders .= "MIME-Version: 1.0\r\n"; 
    $mailheaders .= "Content-Type: multipart/mixed; boundary=\"$boundary\"\r\n"; 

    $multipart  =  "--".$boundary."\r\n";
    $multipart .= "Content-Type: text/plain; \r\n";
    $multipart .= "Content-Transfer-Encoding: 7bit\r\n";    
    $multipart .= "\r\n";
    $multipart .= $message;
    $multipart .= "\r\n";
    
    $multipart .= "--".$boundary."\r\n";
    $fp = fopen($filepath,"r"); 
        if (!$fp) 
        { 
            exit("Can't open file $filepath"); 
        }  

     
    $message_part .= "Content-Type: application/csv; name=\"$filename\" \r\n";  
    $message_part .= "Content-Transfer-Encoding: base64\r\n"; 
    $message_part .= "Content-Disposition: attachment; \r\n"; 

    $message_part .= chunk_split(base64_encode(fread($fp, filesize($filepath))));
    fclose($fp); 

    $message_part .= $boundary."--\r\n";
    $multipart .= $message_part;
     
    mail($to, $subject, $multipart, $mailheaders);