Problem for uploading image with formData, Ajax, and json response [duplicate]

I there,

I will try to be clear, i want to add an input file to a testimonial module of Prestashop.

The JS Code before my intervention was like this :

  $("#testimonialForm").submit(function(event){
event.preventDefault();
var form_data = $(this).serialize();

$.ajax({
  type: 'POST',
  cache: false,
  dataType: 'json',
  url: '',
  data: form_data+'&fc=module&module=csoft_testimonials&controller=addtestimonial&action=addTestimonial&ajax=true',
  success: function(msg){
    console.log(msg);
    switch(msg) {
      case "field_error":
        $(".alert").html(field_error).addClass("alert-danger").slideDown("fast");
        break;
      case "review_exist":
        $(".alert").html(review_exist).addClass("alert-warning").slideDown("fast");
        break;
      case "success":
        $(".alert").html(success).removeClass("alert-danger");
        $(".alert").html(success).addClass("alert-success").slideDown("fast");
        $("#testimonial_submitter_name").val("");
        $("#testimonial_submitter_email").val("");
        $("#testimonial_title").val("");
        $("#testimonial_main_message").val("");
        break;
      case "DB_error":
        $(".alert").html(DB_error).addClass("alert-danger").slideDown("fast");
        break;
      default:
        $(".alert").html(other).addClass("alert-danger").slideDown("fast");
    }
  }
});  });

And after :

 $("#testimonialForm").submit(function(event){
event.preventDefault();

var formData = new FormData($('#testimonialForm')[0]);
formData.append('testimonial_img', $('input[type=file]')[0].files[0]);

$.ajax({
  type: 'POST',
  contentType: false,
  processData: false,
  url: '',
  data: formData+'&fc=module&module=csoft_testimonials&controller=addtestimonial&action=addTestimonial&ajax=true',
  success: function(msg){
    console.log(msg);

    switch(msg) {
      case "field_error":
        $(".alert").html(field_error).addClass("alert-danger").slideDown("fast");
        break;          
      case "img_error":
        $(".alert").html(img_error).addClass("alert-danger").slideDown("fast");
        break;
      case "review_exist":
        $(".alert").html(review_exist).addClass("alert-warning").slideDown("fast");
        break;
      case "success":
        $(".alert").html(success).removeClass("alert-danger");
        $(".alert").html(success).addClass("alert-success").slideDown("fast");
        $("#testimonial_submitter_name").val("");
        $("#testimonial_submitter_email").val("");
        $("#testimonial_title").val("");
        $("#testimonial_main_message").val("");
        break;
      case "DB_error":
        $(".alert").html(DB_error).addClass("alert-danger").slideDown("fast");
        break;
      default:
        $(".alert").html(other).addClass("alert-danger").slideDown("fast");
    }
  }
});  });

As you can see, normally the php script give json answers like : img_error, but now, the "msg" var who is sent to the console log give me the entire html page.

On the php side, this is what I have :

    $order_id = Tools::getValue('id_order');
if (!Validate::isMessage($order_id) && strlen($order_id) == 9) {
  $result = "field_error";
}
$customer_id = Tools::getValue('customer_id');
if (!Validate::isInt($customer_id)) {
  $result = "field_error";
}

$note = Tools::getValue('testimonial_note');
if (!Validate::isInt($note)) {
  $result = "field_error";
}

$message = Tools::getValue('testimonial_message');
if (!Validate::isMessage($message) OR empty($message)) {
  $result = "field_error";
}

$img = $_FILES['testimonial_img'];
$image = $this->_imgDir . $order_id . '-' . $img['name'];
$image_size = file_exists($image) ? filesize($image) / 1000 : false;
if(!$_FILES['testimonial_img']['size'] && !$image_size)
{
  $result = "img_error";
}

if($model->getTestimonialReferenceOrder($order_id)){
  $result = "review_exist";
}

if ($result != "field_error" && $result != "review_exist" && $result != "img_error"){
  $resp = $model->writeTestimonial($order_id, $customer_id, $note, $message, $model->testimonial_img, $id_shop);

  $result = 'success';
}

die(json_encode($result));

If i try to modify the JS file, it's because in the Php side, $_FILES['testimonial_img'] was giving to me an undefined index error.

Can someone explain to me how I can pass the image info into my php script, without losing the json response from the script?