Insertion ajax avec PHP

function insertData(){
  var subject=$("#subject").val();
  var img_sujet=$("#img_sujet").val();
  
  var datas = {subject:subject,img_sujet:img_sujet};
  // AJAX code to send data to php file.
    $.ajax({
        type: "POST",
        url: "subject.php",
        //data: datas,
        data: datas,
        dataType: "JSON",
        success: function(data) {
          $("#message").html(data);
          $("p").addClass("alert alert-success");
        },
        error: function (xhr, ajaxOptions, thrownError) {
          alert(xhr.status);
          alert(thrownError);
        }
    });
  }

onclick

$("p").addClass("alert alert-success");

function insertData(){
  var form = $('form').get(0);
  var formData = new FormData(form);
  $.ajax({
    type		: 'POST', 
    url		: 'subject.php',
    data		: formData, 
    dataType	: 'json',
    processData: false,
    contentType: false,
    success: function(formData) {
      $("#message").html(formData);
      $("p").addClass("alert alert-success");
    },
    error: function (xhr, ajaxOptions, thrownError) {
      alert(xhr.status);
      alert(thrownError);
    }
  });
}

require '../connexion.php';//connexion à la base

$subject = $_POST['subject'];
$date_sujet = date("Y-m-d H:m:s");	
$id_user = $_SESSION['id_user'];
$timeStamp = time();
$imgSujet = $_FILES['img_sujet'];
$sql_insert = "INSERT INTO sujet (id_user, date, texte_sujet) values (:id_user, :date, :texte_sujet)";
    $stmt = $pdo->prepare($sql_insert);
    include 'smiley_encode.php';	
    $nb = $stmt->execute(array(':id_user'=>$id_user, ':date'=>$date_sujet, ':texte_sujet'=>$subject));	
    $last_id = $pdo->lastInsertId();
    
    if (!empty($_FILES['img_sujet']['name'])) {                         
        $nom_fichier = $_FILES['img_sujet']['name'];					
        $ext         = explode('.', $nom_fichier);
        $ext         = strtolower($ext[count($ext) - 1]);
        $nom_md5     = md5($nom_fichier);
        $taille      = $_FILES['img_sujet']['size'];
        $type        = $_FILES['img_sujet']['type'];
        $tmp         = $_FILES['img_sujet']['tmp_name'];
        $chemin      = utf8_encode(stripslashes($nom_fichier));

        $dossier="blog/".$_SESSION['id_user'].''.$timeStamp.''.$last_id.".".$ext.""; 
        if(move_uploaded_file ($_FILES['img_sujet']['tmp_name'][$i], $dossier)) {
            $img_sujet = $_SESSION['id_user'].''.$timeStamp.''.$last_id.".".$ext;	
            $sql_img_sujet = "INSERT INTO img_sujet (id_sujet, id_user, date, img_sujet) values (:id_sujet, :id_user, :date, :img_sujet)";
            $date_img = date("Y-m-d H:m:s");	
            $stmt=$pdo->prepare($sql_img_sujet);
            $nb = $stmt->execute(array(':id_sujet'=>$last_id, ':id_user'=>$id_user, ':date'=>$date_img, ':img_sujet'=>$img_sujet));				
        }
    }

    $array = [$_POST['subject'], $_FILES['img_sujet']];
    // J'indique au navigateur que je retourne du JSON
    header('Content-type: application/json');
    // Je transforme mon tableau en JSON et je l'imprime dans le body de ma réponse
    echo json_encode($array);

<form id="publie" action="subject.php" method="post" enctype="multipart/form-data">
                        <div class="div_flex">
                            <div class="col-1 col-sm-1 col-md-1 img-user">
                                <img src="<?php echo avatar($_SESSION['id_membre']); ?>" class="clipClass">
                            </div>
                            <div class="col-md-9">
                                <div class="form-group">
                                    <textarea class="textarea-subject" name="subject" id="subject" rows="3" placeholder="Exprimez-vous, <?php echo membre_prenom($_SESSION['id_membre']); ?>"></textarea>
                                    <a class="btn_input"><i class="far fa-image" id="ico_image"></i><span class="name">Photos</span></a>
                                    <input type="file" name="img_sujet" id="img_sujet" value="" lang="fr" accept=".jpg,.jpeg,.gif,.png" onchange="loadFile(event)">
                                </div>
                            </div>
                        </div>
                        <div class="image" id="image">
                            <img src="" id="output" class="img-fluid" alt="">
                            <a class="btn_del" onclick="delFile(event)"><i class="fas fa-trash-alt delete" id="trash"></i></a>
                        </div>
                        <div id="testdiv"></div>
                        <hr>
                        <div class="ligne_top"></div>
                        <div>
                           <button type="button" class="btn btn-primary btn-sm publier" id="publier" onclick="insertData()">Publier</button>	
                         
                            
                           
                        </div>
                        <p id="message"></p>
                    </form>