Symfony2 : Save image form field in session - forms

Question

I want to save the form data in session in case of an error occured then I can display valid form data in my form view.
I've got a file field for an image but I've got this error when I submit my form with an image : Serialization of 'Symfony\Component\HttpFoundation\File\UploadedFile' is not allowed
My Form Type :
class ContenuType extends AbstractType
{
private $session;
public function __construct($session)
{
$this->session = $session;
}
/**
* #param FormBuilderInterface $builder
* #param array $options
*/
public function buildForm(FormBuilderInterface $builder, array $options)
{
$session = $this->session;
$formSession = $session->get('dataCreateContenu');
if (!$formSession) {
$formSession = new Contenu();
}
$builder
->add('description', 'textarea', array(
'label' => 'Description',
'data' => $formSession->getDescription(),
'attr' => array('maxlength' => 560),
)
)
->add('file', 'file', array(
'label' => 'Image',
'data' => $formSession->getImage(),
'required' => false,
)
)
;
}
}
My Form Controller :
/**
* New Contenu
*
* #Route("/", name="contenu_create")
* #Method("POST")
* #Template("MyOwnBundle:Contenu:new.html.twig")
*/
public function createAction(Request $request)
{
$entity = new Contenu();
$form = $this->createCreateForm($entity);
$form->handleRequest($request);
$session = $this->get('session');
$session->set('dataCreateContenu', $form->getData());
if ($form->isValid()) {
$em = $this->getDoctrine()->getManager();
$formAll = $form->all();
foreach ($formAll as $formField) {
if(empty($formField->getData())) {
$this->get('session')->getFlashBag()->add('error', 'Field « '.$formField->getConfig()->getOption("label").' » is empty.');
return $this->redirect($this->generateUrl('contenu_new'));
}
}
$image = $form->get('file')->getData();
if ($image) {
$entity->upload();// Upload
}
$em->persist($entity);
$em->flush();
$session->clear();
return $this->redirect($this->generateUrl('contenu_show', array('id' => $entity->getId())));
}
return array(
'entity' => $entity,
'form' => $form->createView(),
);
}
What can I do ?