Php 在symfony（Swiftmailer）中扩展现有电子邮件的最佳方法？

我有一份登记表，在提交时会发送一封电子邮件。现在我的目标是在电子邮件到达用户之前获取它并扩展它。我正在使用基于Symfony的Showpare6

我已经拥有的：

订户：

<?php
namespace RegistrationExtension\Subscriber;

use Shopware\Core\Framework\DataAbstractionLayer\Event\EntityLoadedEvent;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Shopware\Core\Checkout\Customer\CustomerEvents;
use Shopware\Core\Checkout\Customer\CustomerEntity;

class RegisterExtensionSubscriber implements EventSubscriberInterface{
    public static function getSubscribedEvents(): array
    {
        return [
            CustomerEvents::CUSTOMER_REGISTER_EVENT => 'onCustomRegister'
        ];
    }

    public function onCustomRegister(EntityLoadedEvent $event){
        /** @var CustomerEntity $customerRegisEvent */
        foreach ($event->getEntities() as $customerRegisEvent) {
            $customerRegisEvent->addExtension('custom_struct', new RegisterStruct());
        } 
    }
}

我的确切目标是上传一张图片，并将链接作为下载链接放在邮件中

<?php declare(strict_types=1);

namespace RegistrationExtension\Storefront\Controller;

use Shopware\Core\Framework\Routing\Annotation\RouteScope;
use Shopware\Core\System\SalesChannel\SalesChannelContext;
use Shopware\Storefront\Controller\StorefrontController;
use Symfony\Component\Routing\Annotation\Route;
use Symfony\Component\HttpFoundation\JsonResponse;
use Shopware\Core\Framework\DataAbstractionLayer\Search\Criteria;
use Shopware\Core\Framework\DataAbstractionLayer\Search\Filter\EqualsFilter;
use Shopware\Core\Framework\DataAbstractionLayer\EntityRepositoryInterface;
/**
 * @RouteScope(scopes={"storefront"})
*/

class RegistrationExtensionController extends StorefrontController{
    /**
    * @Route("/extension", name="registration.extension", defaults={"XmlHttpRequest"=true})
    */
    public function extendRegisterStorefront(SalesChannelContext $context){
        /**
         * @var EntityRepositoryInterface $mailTypeRepository
         * @var EntityRepositoryInterface $mailRepository
         */
        $mailTypeRepository = $this->container->get('mail_template_type.repository');
        $mailRepository = $this->container->get('mail_template.repository');
        
        $mailEntity = $mailTypeRepository->search(
            (new Criteria())->addFilter(new EqualsFilter('mail_template_type.technicalName', "customer_register")),
            $context->getContext()
        );

        $mailTypeId = $mailEntity->getEntities()->first()->getId();


        $mailEntity = $mailRepository->search(
            (new Criteria())->addFilter(new EqualsFilter('mail_template.mailTemplateTypeId', $mailTypeId)),
            $context->getContext()
        );
        
        //$mailEntity->getEntities()->setContentHtml("test");
        $mail = $mailEntity->getEntities();

        return $this->renderStorefront(
            '@RegistrationExtension/storefront/custom.html.twig',
            [
                'page' => $context,
                'mailEntity' => $mail
            ]
        );
    }
}

<?xml version="1.0" ?>

<container xmlns="http://symfony.com/schema/dic/services"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xsi:schemaLocation="http://symfony.com/schema/dic/services http://symfony.com/schema/dic/services/services-1.0.xsd">

    <services>
        <service id="RegistrationExtension\Storefront\Controller\RegistrationExtensionController" public="true"> </service>
        <service id="RegistrationExtension\Subscriber\RegisterExtensionSubscriber">
            <tag name="kernel.event_subscriber"/>
        </service>
    </services>
</container>