symfony 4 error when uploading file - php

Question

I am trying to upload a file, very much following the instructions on Symfony's cookbook, but it doesn't seem to work.
The specific error is as follows, but the background reason is that the file as such does not seem to be ( or remain ) uploaded.
Call to a member function guessExtension() on string
As it happens, the file is momentarily created at upload_tmp_dir, but gets deleted almost immediately ( I know that 'cause I kept that directory visible on my Finder).
The file metadata is available on the var_dump($_FILES) command on the script below.
So, for some reason the file is being discarded which, I believe, causes the specific error seen above.
I believe $file ( from UploadedFile ), should receive the file as such, not the path to it, but not sure how to get there. Particularly is the file does not remain on upload_tmp_dir.
For information, I tried the upload in a plain PHP project I have and it works fine. The file remains in upload_tmp_dir till is moved elsewhere.
Thanks
Here is the controller:
class ApiUserXtraController extends Controller
{
public function UserXtraAction(Request $request, ValidatorInterface $validator) {
$is_logged = $this->isGranted('IS_AUTHENTICATED_FULLY');
if ($is_logged) {
$user = $this->getUser();
}
$em = $this->getDoctrine()->getManager();
$repo = $em->getRepository(UserXtra::class);
$userxtra = new UserXtra();
$form = $this->createFormBuilder($userxtra)
->add('imgFile', FileType::class, array('label' => 'file'))
->add('save', SubmitType::class, array('label' => 'Create Task'))
->getForm();
var_dump($_FILES); // outputs file metadata, ie, name, type, tmp_name, size
$form->handleRequest($request);
$userxtra->setUser($user);
if ($form->isSubmitted() && $form->isValid()) {
/**
* #var UploadedFile $file
* */
$file = $userxtra->getImgFile();
var_dump('file', $file);// outputs full path to upload_tmp_dir
$fileName = $this->generateUniqueFileName().'.'.$file->guessExtension(); // **THIS THROWS THE ERROR**
$file->move(
$this->getParameter('user_image_directory'),
$fileName
);
$userxtra->setImgFile($fileName);
//$data = json_decode($data);
return new JsonResponse(array(
'status' => 'ok',
'is_logged' => $is_logged,
));
}
return $this->render('upload.html.twig', array(
'form' => $form->createView(),
));
}

Answer 1

Maybe you are looking for something like
$form->getData('imgFile')->guessExtension();
instead?
Edit: Ah sorry, missed that you are assuming that $file = $userxtra->getImgFile(); actually gives back an UploadedFile object. Apparently that assumption is not correct, as the error you are seeing indicates that it gives back a string instead.

Answer 2

I've found the solution on this SO question.
The docs, or actually Symfony's cookbook, is wrong.
The line on my code above that states:
$file = $userxtra->getImgFile();
should be:
$file = $form->get('imgFile')->getData();