The form view data is expected to be an instance of the class ... but is string (n)

Question

I am currently getting the following error:

"It is expected that the data of the form view will be an instance of the class Symfony \ Component \ HttpFoundation \ File \ File, but they are the string (n). You can avoid this error by setting the" data_class "option equal to or adding a form transformer that converts the string (n) to an instance of Symfony \ Component \ HttpFoundation \ File \ File. "

SoundController - download function

 /**
 * @Security("is_granted('IS_AUTHENTICATED_FULLY')")
 * @Route("/song/upload", name="upload_song")
 * @param Request $request
 * @return \Symfony\Component\HttpFoundation\Response
 */
public function uploadSong(Request $request)
{
    $song = new Sound();

    $form = $this->createForm(SoundType::class, $song);
    $form->handleRequest($request);

    if ($form->isSubmitted() && $form->isValid())
    {
        $file = $song->getFile();
        $user = $this->getUser();

        $fileName = $this
            ->get('app.file_uploader')
            ->setDir($this->get('kernel')->getRootDir()."/../web".$this->getParameter('songs_directory'))
            ->upload($file);

        $song->setFile($fileName);

        $file = $song->getCoverFile();
        if ($file === null)
        {
            $song->setCoverFile($this->getParameter('default_cover'));
        }
        else
        {
            $fileName = $this
                ->get('app.file_uploader')
                ->setDir($this->get('kernel')->getRootDir()."/../web".$this->getParameter('covers_directory'))
                ->upload($file);

            $song->setCoverFile($fileName);
        }

        $song->setUploader($user);
        $song->setUploaderID($user->getId());
        $user->addSong($song);

        $entityManager = $this->getDoctrine()->getManager();
        $entityManager->persist($song);
        $entityManager->flush();

        return $this->redirectToRoute('song_view', [
            'id' => $song->getId()
        ]);
    }

    return $this->render('song/upload.html.twig', [
        'form' => $form->createView()
    ]);
}

SoundType - Form

public function buildForm(FormBuilderInterface $builder, array $options)
{
    $builder
        ->add('file', FileType::class)
        ->add('coverFile', FileType::class, [
            'required' => false
        ])
        ->add('songName', TextType::class)
        ->add('songAuthor', TextType::class);
}

Answer 1

Here is the answer:

{
      $builder
          ->add('file', FileType::class, array('data_class' => null))
          ->add('coverFile', FileType::class, array('data_class' => null))
          ->add('coverFile', FileType::class, array('data_class' => null,'required' => false))
          ->add('songName', TextType::class)
          ->add('songAuthor', TextType::class);
  }

Answer 2

/**
 * @ORM\Column(type="string")
 *
 * @Assert\NotBlank(message="Please, upload the song as a MP3 file.")
 * @Assert\File(mimeTypes={ "audio/mpeg", "audio/wav", "audio/x-wav", "application/octet-stream" })
 */
private $file;

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