Symfony2: Transform ArrayCollection to File

Question

I get an error when trying to store a form with multiple file input.

I have two entities: Inquiry and UploadedDocumend which are in a one-to-many relation.

Here is the transformer for the uploaded files:

public function reverseTransform($uploadedDocuments)
{
    $attachments = array();
    foreach($uploadedDocuments as $file){
        if($file) {
            $uploadedDocument = new UploadedDocument();
            /* @var $uploadedDocument UploadedDocument */
            $uploadedDocument->setFile($file);
            $uploadedDocument->setCreatedOn(new \DateTime("now"));
            $this->em->persist($uploadedDocument);
            $attachments[] = $uploadedDocument;
        }
    }
    return $attachments;
}

And here the code to add the transformer:

->add($builder->create(
                'uploadedDocuments', 'file', [
                    'required' => false,
                    'multiple' => true,
                    'label' => "Anhänge"
            ]
        )->addModelTransformer(new UploadsTransformer($this->em)))

With this I get the following exception:

The form's view data is expected to be an instance of class Symfony\Component\HttpFoundation\File\File, but is an instance of class Doctrine\Common\Collections\ArrayCollection. You can avoid this error by setting the "data_class" option to null or by adding a view transformer that transforms an instance of class Doctrine\Common\Collections\ArrayCollection to an instance of Symfony\Component\HttpFoundation\File\File.

I can't change the resolver. Because I need it that way. I think.

public function setDefaultOptions(OptionsResolverInterface $resolver) {
    $resolver->setDefaults(array(
        'data_class' => 'Lengoo\FormhandlerBundle\Entity\Inquiry',
    ));
}

How do I have to change the transformer so the object is created correctly? FYI: It works if I just store the path from the file to an array. But I want the relation between the two objects.