MediaType as a child form type

src\Block\AbstractMediaUploaderBlock.php

<?php

declare(strict_types=1);

namespace App\Block;

use App\Entity\SonataMediaMedia;
use App\Traits\EntityManagerTrait;
use Sonata\AdminBundle\Form\FormMapper;
use Sonata\BlockBundle\Block\Service\AbstractAdminBlockService;
use Sonata\BlockBundle\Model\BlockInterface;

abstract class AbstractMediaUploaderBlock extends AbstractAdminBlockService
{
    use EntityManagerTrait;

    protected $medias = [];

    public function buildEditForm(FormMapper $formMapper, BlockInterface $block): void
    {
        $this->loadMedias($block);
    }

    public function prePersist(BlockInterface $block): void
    {
        parent::prePersist($block);

        $this->saveMedias($block);
    }

    public function preUpdate(BlockInterface $block): void
    {
        parent::preUpdate($block);

        $this->saveMedias($block);
    }

    protected function loadMedias(BlockInterface $block): void
    {
        $repository = $this->em->getRepository(SonataMediaMedia::class);
        foreach ($this->medias as $name) {
            $file = $block->getSetting($name);
            if (null !== $file) {
                if (is_numeric($file)) {
                    $file = $repository->find($file);
                }
                if (!$file instanceof SonataMediaMedia) {
                    $file = null;
                }
                $block->setSetting($name, $file);
            }
        }
    }

    protected function saveMedias(BlockInterface $block): void
    {
        foreach ($this->medias as $name) {
            $file = $block->getSetting($name);
            if ($file instanceof SonataMediaMedia) {
                if ($file->getSize()) {
                    $this->em->persist($file);
                    $this->em->flush();

                    $block->setSetting($name, $file->getId());
                }
            }
        }
    }
}

src\Block\TextBannerBlock.php

<?php

declare(strict_types=1);

namespace App\Block;

use App\Form\Type\LayoutFormType;
use Sonata\AdminBundle\Form\FormMapper;
use Sonata\BlockBundle\Block\BlockContextInterface;
use Sonata\BlockBundle\Model\BlockInterface;
use Sonata\CoreBundle\Form\Type\ImmutableArrayType;
use Sonata\CoreBundle\Model\Metadata;
use Sonata\FormatterBundle\Form\Type\FormatterType;
use Sonata\MediaBundle\Admin\BaseMediaAdmin;
use Sonata\MediaBundle\Form\Type\MediaType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\OptionsResolver\OptionsResolver;

class TextBannerBlock extends AbstractMediaUploaderBlock
{
    protected $medias = ['background'];

    /**
     * @var BaseMediaAdmin
     */
    private $mediaAdmin;

    public function execute(BlockContextInterface $blockContext, Response $response = null)
    {
        return $this->renderResponse($blockContext->getTemplate(), [
            'block' => $blockContext->getBlock(),
            'settings' => $blockContext->getSettings(),
        ], $response);
    }

    public function buildEditForm(FormMapper $formMapper, BlockInterface $block): void
    {
        parent::buildEditForm($formMapper, $block);

        $fieldDescription = $this->mediaAdmin->getModelManager()->getNewFieldDescriptionInstance(
            $this->mediaAdmin->getClass(), 'media', ['translation_domain' => 'SonataMediaBundle']
        );
        $fieldDescription->setAdmin($formMapper->getAdmin());

        $formMapper->add('settings', ImmutableArrayType::class, [
            'keys' => [
                ['content', FormatterType::class, function (FormBuilderInterface $formBuilder) {
                    return [
                        'event_dispatcher' => $formBuilder->getEventDispatcher(),
                        'format_field' => ['format', '[format]'],
                        'source_field' => ['rawContent', '[rawContent]'],
                        'target_field' => '[content]',
                        'label' => 'form.label_content',
                    ];
                }],
                ['background', MediaType::class, [
                    'sonata_field_description' => $fieldDescription,
                    'provider' => 'sonata.media.provider.image',
                    'required' => false,
                    'context' => 'banners',
                ]],
                ['layout', LayoutFormType::class, [
                    'required' => false,
                ]],
            ],
            'translation_domain' => 'SonataFormatterBundle',
        ]);
    }

    public function configureSettings(OptionsResolver $resolver): void
    {
        $resolver->setDefaults([
            'format' => 'richhtml',
            'rawContent' => '<b>Insert your custom content here</b>',
            'content' => '<b>Insert your custom content here</b>',
            'background' => null,
            'layout' => [],
            'template' => 'Block/text_banner.html.twig',
        ]);
    }

    public function getBlockMetadata($code = null): Metadata
    {
        return new Metadata(
            $this->getName(), $code ?? $this->getName(), false, 'AppBlocks', [
                'class' => 'fa fa-file-text-o',
            ]
        );
    }

    /**
     * @required
     */
    public function setMediaAdmin(BaseMediaAdmin $mediaAdmin): void
    {
        $this->mediaAdmin = $mediaAdmin;
    }
}

src\Traits\EntityManagerTrait.php

<?php

declare(strict_types=1);

namespace App\Traits;

use Doctrine\ORM\EntityManagerInterface;

trait EntityManagerTrait
{
    /**
     * @var EntityManagerInterface
     */
    protected $em;

    /**
     * @required
     */
    public function setEntityManager(EntityManagerInterface $em): void
    {
        $this->em = $em;
    }
}