mail() - Allegato arriva in formato testo

[camionistaxcaso]

Ciao, invio la mail in PHP con "mail()", inserendo un allegato però la mail arriva, l'allegato pure, ma arriva in formato testo dentro la pagina del browser dove leggo la mail.
Qualcuno mi aiuta? Grazie a tutti.

//QUESTA PARTE DI CODICE INVIA LA MAIL
// Recupero il valore dei campi del form
$destinatario = $_POST['chooseaddress'];
$mittente = $_POST['mitt'];
$oggetto = $_POST['oggetto'];
$messaggio = $_POST['msg'];

// ASSEGNIAMO A VARIABILI PIU' LEGGIBILI, LE PROPRIETA' DELL'ALLEGATO
$attach = $_FILES["allegato"]["tmp_name"];
$file_name = $_FILES["allegato"]["name"];
$file_type = $_FILES["allegato"]["type"];
$file_size = $_FILES["allegato"]["size"];

// DELIMITATORE
$boundary = md5(uniqid(microtime()));

// APRIAMO L'ALLEGATO PER LEGGERLO E CODIFICARLO
$file = @fopen($attach, "r");
$contents = @fread($file, $file_size);
$encoded_attach = chunk_split(base64_encode($contents));
@fclose($file);

// INTESTAZIONI DELLA MAIL
$mail_headers .= "MIME-version: 1.0\n";
$mail_headers .= "Content-type: multipart/mixed; boundary=\"$boundary\"";
$mail_headers .= "X-attachments: $file_name\n";
$mail_headers .= "From: <$mittente>\r\n";

// COSTRUIAMO IL CORPO DELLA MAIL
$mail_body = "--$boundary\n";
$mail_body .= "Content-Type: text/plain; charset=us-ascii\n";
$mail_body .= "Content-Transfer-Encoding: 7bit\n\n";
$mail_body .= "$messaggio\n\n";
$mail_body .= "--$boundary\n";
$mail_body .= "Content-type: $file_type; name=\"$file_name\"\n";
$mail_body .= "Content-Transfer-Encoding: base64\n";
$mail_body .= "$encoded_attach\n";
$mail_body .= "--$boundary--\n"; 

// Invio la mail
if (mail($destinatario, $oggetto, $mail_body, $mail_headers)) 
   {
     $msg = "<div style='margin-left:5px; color:#00CC00'>Messaggio inviato con successo.</div>

";
   }
else
   {
     $msg = "<div style='margin-left:5px; color:FF0000'>Errore: 
Si è verificato un errore durante l'invio della mail.</div>

";
   }
         
//FINE INVIO EMAIL 

In posta mi arriva questo:

Mail da blackskip.com Posta in arrivo 	x asd@asd.it tramite webxc02s08.ad.aruba.it  	 22:19 (10 minuti fa) 		 a me --a33c07dc7fb487f895f9ca6ad771643e Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit  Testo di prova Testo di prova Testo di prova Testo di prova Testo di prova Testo di prova Testo di prova Testo di prova Testo di prova Testo di prova Testo di prova Testo di prova Testo di prova  --a33c07dc7fb487f895f9ca6ad771643e Content-type: text/plain; name="Ordinanza catene a bordo.txt" Content-Transfer-Encoding: base64 TWFzc2EsIDEgZGljZW1icmUgMjAxMSAtIEyST1JESU5BTlpBIGRlbGxhIFBvdmluY2lhIGRpIE1h c3NhIENhcnJhcmEsIGNoZSBtb2RpZmljYSBsYSBjYXJhdHRlcmlzdGljYSBkZWxsZSBjYXRlbmUg ZGEgbmV2ZSBlIGRlaSBwbmV1bWF0aWNpIGludmVybmFsaSBkYSBhY2Nlc3NvcmkgYSBkaXNwb3Np dGl2aSBkaSBzaWN1cmV6emEsIGhhIHNjYXRlbmF0byBsZSBwcm90ZXN0ZSBkZWkgbm9zdHJpIGxl dHRvcmkgY2hlIGRhbCAxNSBub3ZlbWJyZSwgZmlubyBhIHR1dHRvIGlsIDMxIG1hcnpvIDIwMTIs 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Ciao e grazie a tutti

PS: il sito si trova su piattaforma Apache/2.2