plugins - wp_handle_upload - specified file failed upload test

I'm using the wp_handle_upload() function to create a plugin that will give to the users the ability to upload files from front-end using ajax. I've solved an issue relative to an error in the syntax, now while I'm debugging the code I get this error from the function: specified file failed upload test. I don't know what's wrong with the code, can anyone help me?

PHP

  public function upload_form( $attr, $content = null )
  {
    ob_start();
    ?>
      <form method="POST" enctype="multipart/form-data">
        <div class="custom-file">
          <input type="file" name="imageFile" class="custom-file-input" id="imageFile" accept="image/*">
          <label class="custom-file-label" for="imageFile">Choose file</label>
        </div>
        <textarea class="form-control" id="imageMessage" name="message"></textarea>
        <button type="button" class="btn btn-outline-secondary" id="process-upload"><?php _e('Salva'); ?></button>
        <button type="reset" class="btn btn-outline-danger" id="cancel-upload"><?php _e('Annulla'); ?></button>
      </form>
    <?php
    return ob_get_clean();
  }

  public function process_upload()
  {
    // $file = array(
    //   'name' => $_FILES['imageFile']['name'],
    //   'type' => $_FILES['imagefile']['type'],
    //   'tmp_name' => $_FILES['imageFile']['tmp_name'],
    //   'size' => $_FILES['imageFile']['size'],
    //   'error' => $_FILES['imageFile']['error'],
    // );

    $upload = wp_handle_upload( $_FILES['imagefile'] , array( 'test_form' => false ) );

    var_dump($upload);

  }

in my js script I have this code: NB: on the form I've specified the enctype and the POST method

$('#process-upload').on('click', function(e){
      e.preventDefault();
      var data = new FormData();
      data.append('action', 'new_message');
      data.append('imageFile', $('#imageFile').prop('files')[0] );
      data.append('message', $('#imageMessage').val() );

      $.ajax({
        url: wpu.ajaxurl,
        type: 'POST',
        contentType: false,
        processData: false,
        data: data,
        success: function(response){
          console.log(response);
          window.alert('Done!');
        }
      });

    });

HTML form

<form method="POST" enctype="multipart/form-data">
        <div class="custom-file">
          <input type="file" name="imageFile" class="custom-file-input" id="imageFile" accept="image/*">
          <label class="custom-file-label" for="imageFile">Choose file</label>
        </div>
        <textarea class="form-control" id="imageMessage" name="message"></textarea>
        <button type="button" class="btn btn-outline-secondary" id="process-upload"><?php _e('Save'); ?></button>
        <button type="reset" class="btn btn-outline-danger" id="cancel-upload"><?php _e('Cancel'); ?></button>
      </form>