use SymfonyComponentValidatorConstraints as Assert;

 /**
 * @ORMColumn(type="string", length=255, unique=true)
 * @AssertFile(
 *     maxSize = "1024k",
 *     mimeTypes = {"application/pdf", "application/x-pdf"},
 *     mimeTypesMessage = "Please upload a valid PDF")
 */
private $file;

/**
 * @ORMManyToOne(targetEntity="AppEntityTicket", inversedBy="files")
 */
private $ticket;

public function getFile(): ?string
{
    return $this->file;
}

public function setFile(string $file): self
{
    $this->file = $file;

    return $this;
}

public function getTicket(): ?Ticket
{
    return $this->ticket;
}

public function setTicket(?Ticket $ticket): self
{
    $this->ticket = $ticket;

    return $this;
}

use SymfonyComponentValidatorConstraints as Assert;

/**
 * @ORMOneToMany(targetEntity="AppEntityTicketFile", mappedBy="ticket")
 * @AssertValid()
 */
private $files;

/**
 * @return Collection|TicketFile[]
 */
public function getFiles(): Collection
{
    return $this->files;
}

public function addFile(TicketFile $file): self
{
    if (!$this->files->contains($file)) {
        $this->files[] = $file;
        $file->setTicket($this);
    }

    return $this;
}

public function removeFile(TicketFile $file): self
{
    if ($this->files->contains($file)) {
        $this->files->removeElement($file);
        // set the owning side to null (unless already changed)
        if ($file->getTicket() === $this) {
            $file->setTicket(null);
        }
    }

    return $this;
}

use SymfonyComponentFormExtensionCoreTypeTextareaType;
use SymfonyComponentValidatorConstraintsLength;
use SymfonyComponentFormExtensionCoreTypeFileType;

$builder
        ->add('description', TextareaType::class, [
            'label' => 'Opis zgłoszenia',
            'constraints' => new Length(['min' => 50]),
        ])
        ->add('files', FileType::class, [
            'multiple' => true,
            'label' => 'Załączniki',
        ])
        ->add('save', SubmitType::class);

public function addTicket(Request $request)
{
    $form = $this->createForm(TicketType::class);

    $form->handleRequest($request);

    if ($form->isSubmitted() && $form->isValid()) {
        $this->ticketService->saveTicket($form->getData());
        return $this->redirectToRoute('add_ticket');
    }

    return $this->render('user/add_ticket.html.twig', [
        'form' => $form->createView(),
    ]);
}

public function saveTicket($data, $user = null) {
    $statusNew = $this->em->getRepository(Status::class)->findOneBy(['id' => Status::STATUS_NEW]);

    $ticket = new Ticket();
    $ticket
        ->setUser($user)
        ->setDescription($data['description'])
        ->setStatus($statusNew);

    if ($user) {
        $ticket
            ->setUser($user)
            ->setProject($data['project'])
            ->setProjectName($data['project']->getName())
            ->setRequestEmail($user->getEmail());
    } else {
        $ticket
            ->setRequestEmail($data['requestEmail'])
            ->setProjectName($data['projectName']);
    }

    $this->em->persist($ticket);

    foreach ($data['files'] as $file) {
        $extension = $file->getClientOriginalExtension();

        $filename = md5(uniqid()).'.'.$extension;
        $file->move('upload/ticketFiles/', $filename);

        $ticketFile = new TicketFile();
        $ticketFile
            ->setFile($filename)
            ->setTicket($ticket);
        $this->em->persist($ticketFile);
    }

    $this->em->flush();
}