Upload multiple files form - Symfony 5

Question

I have a multiple files upload working like a charm in EasyAdmin. But from the front-end side it doesn't want to upload files at all. I can see previously uploaded files and I see the "upload file" button, but after clicking save it gives me: "App\Entity\Uzytkownicy::addZalaczniki(): Argument #1 ($zalaczniki) must be of type App\Entity\Zalaczniki, Doctrine\ORM\PersistentCollection given, called in XXX\Controller\DefaultController.php on line 103"

in DefaultController.php

$uploadForm = $this->createForm(ZalacznikiKlientFormType::class, $user);
        $uploadForm->handleRequest($req);

        if ($uploadForm->isSubmitted() && $uploadForm->isValid()) {
 ***THIS IS LINE 103***->>           $user->addZalaczniki($uploadForm->get('zalacznikis')->getData());
            $entityManager = $this->getDoctrine()->getManager();
            $entityManager->persist($user);
            $entityManager->flush();

            try {$mailer->send($email);
            } catch (TransportExceptionInterface $e) {
            }
        }

ZalacznikiKlientFormType.php

<?php

namespace App\Form;

use App\Entity\Uzytkownicy;
use App\Entity\Zalaczniki;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\CollectionType;
use Symfony\Component\Form\Extension\Core\Type\SubmitType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;
use Vich\UploaderBundle\Form\Type\VichFileType;

class ZalacznikiKlientFormType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options): void
    {
        $builder
            ->add('zalacznikis', CollectionType::class, [
                'entry_type' => ZalacznikiFormType::class,
                'allow_add' => true,
                'entry_options' => ['label' => false, 'allow_file_upload' => true],
                'allow_delete' => true,
                'required' => false,
                'label'=> true,
                'by_reference' => false,
                'disabled' => false,
            ])

            ->add('Zapisz', SubmitType::class, [
            'attr' => ['class' => 'btn btn-primary mt-1']])

        ;
    }

    public function configureOptions(OptionsResolver $resolver): void
    {
        $resolver->setDefaults([
            'data_class' => Uzytkownicy::class,
        ]);
    }
}

ZalacznikiFormType.php

<?php

namespace App\Form;

use App\Entity\Zalaczniki;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;
use Vich\UploaderBundle\Form\Type\VichFileType;

class ZalacznikiFormType extends AbstractType

    {
        public function buildForm(FormBuilderInterface $builder, array $options): void
        {
            $builder
                ->add('File', VichFileType::class, array(
                    'label' => 'Plik',
                ))
            ;
        }
    
        public function configureOptions(OptionsResolver $resolver): void
        {
            $resolver->setDefaults([
                'data_class' => Zalaczniki::class,
            ]);
        }
    }

I can't find helpful information anywhere. I've found this How to upload multiple files with vich uploader in Symfony 5 but right now I'm hitting the wall.