php javascript ajax file upload not working

I'm using form data for uploading files code given below

var formData = new FormData();
/* Add the file */ 
formData.append("qqfile", file);
xhr.open("post", 'upload.php', true);
xhr.setRequestHeader("Content-Type", "application/octet-stream");
xhr.send(formData);  /* Send to server */ 

in upload.php my code is given below

$fileReader = fopen('php://input', "r");  
$fileWriter = fopen($this->_destination_file, "w+");  

while(true) {  
    $buffer = fgets($fileReader, 4096);
    if (strlen($buffer) == 0) {  
        fclose($fileReader);  
        fclose($fileWriter);  
        return true;  
    }  

    fwrite($fileWriter, $buffer);  
} 
return false;  

when i'm trying to upload pdf file it's working perfectly, when I'm trying to upload .xls file,file is uploaded but when i open xls file getting non readable character.