The file "" does not exist after editing the form with file upload

I'm trying to do file upload for my Task entity. I working with two sources: http://symfony.com/doc/3.4/controller/upload_file.html and Symfony2 file upload step by step but I can't figure out how to keep uploaded file during editing.

I'm not sure if I implemented right the part:

My form was complaining when I tried to edit any Task:

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

So I modified getter getBrochure() in my Task entity which I want to expand of PDF file. Please, see below my getBrochure method This is code of my entity:

<?php

namespace AppBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Validator\Constraints as Assert;
use Symfony\Component\HttpFoundation\File\File;

/**
 * Task
 *
 * @ORM\Table(name="task")
 * @ORM\Entity(repositoryClass="AppBundle\Repository\TaskRepository")
 */
class Task
{
    /**
     * @var int
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @var string
     *
     * @ORM\Column(name="name", type="string", length=255)
     */
    private $name;

    /**
     * @var \DateTime
     *
     * @ORM\Column(name="datetime", type="datetime")
     */
    private $datetime;

    /**
     * @ORM\ManyToMany(targetEntity="Category", inversedBy="tasks")
     * @ORM\JoinTable(name="categories_tasks")
     */
    private $categories;

    /**
     * @ORM\Column(type="text")
     */
    private $description;

    /**
     * @ORM\Column(type="string")
     *
     * @Assert\File(mimeTypes={ "application/pdf" })
     */
     private $brochure;


    public function __construct() {
        $this->categories = new \Doctrine\Common\Collections\ArrayCollection();
    }

    /**
     * Get id
     *
     * @return int
     */
    public function getId()
    {
        return $this->id;
    }

    /**
     * Set name
     *
     * @param string $name
     *
     * @return Task
     */
    public function setName($name)
    {
        $this->name = $name;

        return $this;
    }

    /**
     * Get name
     *
     * @return string
     */
    public function getName()
    {
        return $this->name;
    }

    /**
     * Set datetime
     *
     * @param \DateTime $datetime
     *
     * @return Task
     */
    public function setDatetime($datetime)
    {
        $this->datetime = $datetime;

        return $this;
    }

    /**
     * Get datetime
     *
     * @return \DateTime
     */
    public function getDatetime()
    {
        return $this->datetime;
    }

    public function getCategories()
    {
        return $this->categories;
    }

    public function setCategories(Category $categories)
    {
        $this->categories = $categories;
    }

    public function getDescription()
    {
        return $this->description;
    }

    public function setDescription($description)
    {
        $this->description = $description;
    }

    public function getBrochure()
    {
        //return $this->brochure;
        return new File($this->brochure);
    }

    public function setBrochure($brochure)
    {
        $this->brochure = $brochure;

        return $this;
    }

    public function __toString() {
        return $this->name;
    }
}

?>

The result is that I can load edit page, but the field of file upload is empty, there is no information that I uploaded any file. I'm not sure if there should be any information but I see in database that the filename is there and also in web folder there is uploaded file. When I change anything in Task and save of file is cleared and when I try to launch edit page I see:

The file "" does not exis

What is clear for me, because file column for this Task was cleared. So, how to keep file during editing when I don't want to upload new file?

This is my TaskType

<?php

namespace AppBundle\Form;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;
use Ivory\CKEditorBundle\Form\Type\CKEditorType;
use Symfony\Component\Form\Extension\Core\Type\FileType;

class TaskType extends AbstractType
{
    /**
     * {@inheritdoc}
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->add('name')->add('datetime')->add('categories')
            ->add('description', 'Ivory\CKEditorBundle\Form\Type\CKEditorType', array())
            ->add('brochure', FileType::class, array('label' => 'Broszurka (PDF)', 'required' => false));
    }

    /**
     * {@inheritdoc}
     */
    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => 'AppBundle\Entity\Task'
        ));
    }

    /**
     * {@inheritdoc}
     */
    public function getBlockPrefix()
    {
        return 'appbundle_task';
    }


}

and this is my TaskController (only edit action)

/**
 * Displays a form to edit an existing task entity.
 *
 * @Route("/{id}/edit", name="task_edit")
 * @Method({"GET", "POST"})
 */
public function editAction(Request $request, Task $task)
{
    $deleteForm = $this->createDeleteForm($task);
    $editForm = $this->createForm('AppBundle\Form\TaskType', $task);
    $editForm->handleRequest($request);

    if ($editForm->isSubmitted() && $editForm->isValid()) {
        $this->getDoctrine()->getManager()->flush();

        return $this->redirectToRoute('task_edit', array('id' => $task->getId()));
    }

    return $this->render('task/edit.html.twig', array(
        'task' => $task,
        'edit_form' => $editForm->createView(),
        'delete_form' => $deleteForm->createView(),
    ));
}

Task Entity:

<?php

namespace AppBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Validator\Constraints as Assert;
use Symfony\Component\HttpFoundation\File\File;

/**
 * Task
 *
 * @ORM\Table(name="task")
 * @ORM\Entity(repositoryClass="AppBundle\Repository\TaskRepository")
 */
class Task
{
    /**
     * @var int
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @var string
     *
     * @ORM\Column(name="name", type="string", length=255)
     */
    private $name;

    /**
     * @var \DateTime
     *
     * @ORM\Column(name="datetime", type="datetime")
     */
    private $datetime;

    /**
     * @ORM\ManyToMany(targetEntity="Category", inversedBy="tasks")
     * @ORM\JoinTable(name="categories_tasks")
     */
    private $categories;

    /**
     * @ORM\Column(type="text")
     */
    private $description;

    /**
     * @ORM\Column(type="string")
     *
     * @Assert\File(mimeTypes={ "application/pdf" })
     */
     private $brochure;


    public function __construct() {
        $this->categories = new \Doctrine\Common\Collections\ArrayCollection();
    }

    /**
     * Get id
     *
     * @return int
     */
    public function getId()
    {
        return $this->id;
    }

    /**
     * Set name
     *
     * @param string $name
     *
     * @return Task
     */
    public function setName($name)
    {
        $this->name = $name;

        return $this;
    }

    /**
     * Get name
     *
     * @return string
     */
    public function getName()
    {
        return $this->name;
    }

    /**
     * Set datetime
     *
     * @param \DateTime $datetime
     *
     * @return Task
     */
    public function setDatetime($datetime)
    {
        $this->datetime = $datetime;

        return $this;
    }

    /**
     * Get datetime
     *
     * @return \DateTime
     */
    public function getDatetime()
    {
        return $this->datetime;
    }

    public function getCategories()
    {
        return $this->categories;
    }

    public function setCategories(Category $categories)
    {
        $this->categories = $categories;
    }

    public function getDescription()
    {
        return $this->description;
    }

    public function setDescription($description)
    {
        $this->description = $description;
    }

    public function getBrochure()
    {
        //return $this->brochure;
        return new File($this->brochure);
    }

    public function setBrochure($brochure)
    {
        $this->brochure = $brochure;

        return $this;
    }

    public function __toString() {
        return $this->name;
    }
}

?>

Solution

OK I found the problem few seconds after I wrote comment and added Task entity code. In getBrochure method I tried to create File object for every Task instance even it hasn't any brochure so the solution is to use:

public function getBrochure()
{
    return $this->brochure;
    //return new File($this->brochure);
}