Command.php

<?php

declare(strict_types=1);

...

use Symfony\Component\Validator\Constraints as Assert;

class Command
{
    /**
     * @var string
     *
     * @Assert\NotBlank(message="Нельзя создать пустой тег")
     */
    public $name;

    /**
     * @var string
     *
     * @Assert\NotBlank(message="У тега должна быть картинка.")
     */
    public $image;
}

Form.php

<?php
...

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\FileType;
use Symfony\Component\Form\Extension\Core\Type\TextType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;

class Form extends AbstractType
{
    /**
     * @param FormBuilderInterface $builder
     * @param array $options
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('name', TextType::class, ['label' => 'Имя'])
            ->add('image', FileType::class, ['label' => 'Изображение']);
    }

    /**
     * @param OptionsResolver $resolver
     */
    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults([
            'data_class' => Command::class
        ]);
    }
}

Handler.php

<?php
..

class Handler
{
    /**
     * @var EntityManagerInterface
     */
    private $em;
    /**
     * @var TagRepository
     */
    private $tags;

    public function __construct(EntityManagerInterface $em, TagRepository $tagsRepository)
    {

        $this->em = $em;
        $this->tags = $tagsRepository;
    }

    /**
     * @param Command $command
     * @throws \Exception
     */
    public function handle(Command $command): void
    {
        if ($this->tags->findOneBy(['name' => $command->name])) {
            throw new \DomainException('Такой тег уже существует');
        }

        $tag = new Tag($command->name, $command->image);

        $this->tags->add($tag);
        $this->em->flush();
    }
}

TagsController.php

<?php

...

class TagController extends AbstractController
{
    public function create(Request $request, Create\Handler $handler, FileUploader $fileUploader): Response
    {
        $command = new Create\Command();

        $form = $this->createForm(Create\Form::class, $command);
        $form->handleRequest($request);

        if ($form->isSubmitted() && $form->isValid()) {
            /** @var UploadedFile $file */
            $file = $form->get('image')->getData();
            $uploadedFile = $fileUploader->upload($file, self::DIR);

            $command->image = $uploadedFile->getFullName();

            try {
                $handler->handle($command);
                $this->flashMessageService->success('Тег создан');
                return $this->redirectToRoute('admin_tags_index');
            } catch (\DomainException $exception) {
                $this->flashMessageService->error($exception->getMessage());
                return $this->redirectToRoute('admin_tags_index');
            }
        }

        return $this->render('admin/work/management/tags/create.html.twig', [
            'form' => $form->createView()
        ]);
    }
}