I have trouble to read the file on Symfony 4. The file is read as a string, not as the File type. I understand this from this error response.

$fileName = $this->generateUniqueFileName().'.'.$file->guessExtension();

This line returns as

Call to a member function guessExtension() on string

But I set the types and everything perfectly :

my controller part for reading the values

$certificate = new Certificate();
$form = $this->createForm(CertificateType::class, $certificate);

$form->handleRequest($request);

if ($form->isSubmitted() && $form->isValid()) {

// $file stores the uploaded PDF file
/**
 * @var UploadedFile $file
 */
$file = $certificate->getCertificatePdf();

$fileName = $this->generateUniqueFileName().'.'.$file->guessExtension();
// moves the file to the directory where brochures are stored
$file->move(
    $this->getParameter('brochures_directory'),
    $fileName
);

This is My CertificateType class

$builder->add('certificatePdf', FileType::class);

My Entity Class

/**
 * @ORM\Column(type="string")
 *
 * @Assert\NotBlank(message="Please, upload the verified certificate  as a PDF file.")
 * @Assert\File(mimeTypes={ "application/pdf" })
 */
private $certificatePdf;

public function getCertificatePdf()
{
return $this->certificatePdf;
}

public function setCertificatePdf(string $certificatePdf): self
{
$this->certificatePdf = $certificatePdf;

return $this;
}

I follow the Symfony 4.1 Official document to create it. But I don't know why it is not working. Link for the Documentation