AJAX telechargement de fichiers

function download_answer(file){
var xhr_object = null;
	
if  (window.XMLHttpRequest){ // Firefox 
	xhr_object = new XMLHttpRequest();
	// Handling Safari/firefox bug :
	if (xhr_object.overrideMimeType) {
		xhr_object.overrideMimeType("text/html");
		//xhr_object.overrideMimeType("text/xml");
	}
}
else if(window.ActiveXObject) // Internet Explorer 
	xhr_object = new ActiveXObject("Microsoft.XMLHTTP"); 
else { // XMLHttpRequest non supporté par le navigateur 
	alert("Votre navigateur ne supporte pas les objets XMLHTTPRequest..."); 
return; 
}
	
var filename = "telecharger_reponse.php?fiche="+ file;

xhr_object.open("GET", filename, true); 

xhr_object.onreadystatechange = function() { 
	//alert('wait');
	if (xhr_object.readyState == 4) {
		if (xhr_object.status == 200) {
			var tmp = xhr_object.responseText;
			if(tmp == "ok"){
				alert("Ok");
			}else{
				alert("Un problème est survenu.");
			}
		}else{ 
			alert('Un problème est survenu.');
		} 
	}
}
xhr_object.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
xhr_object.send("NULL");

return(false);
}

$fichier = 'Reponses/'.$_SESSION['id_sondage'].'_'.$_GET['fiche'].'_reponse.csv';
$fp = fopen($fichier, 'w') or die("can't open file");

$txt = array();
$txt2 = array();
foreach($reponse as $r){
	$question = $req->get_question_by_id($r['id_question']);
	$txt[] = $question['titre_q'];
	str_replace("__", " | ", $r['text_reponse']);
	$txt2[] = $r['text_reponse'];
}
	
fputcsv($fp, $txt, ";");
fputcsv($fp, $txt2, ";");
	
fclose($fp);
	
header('Content-Transfer-Encoding: binary'); 
header('Content-Disposition: attachment; filename="reponse_'.$_GET['fiche'].'.csv"');  
  
readfile($fichier);