如何在Symfony 4中设置上传文件字段的可空约束？

composer require symfony/form

use Symfony\Component\Validator\Constraints as Assert;

class YourEntity
{
    /**
     * @Assert\File(
     *     maxSize = "5M",
     *     mimeTypes = {"image/jpeg", "image/png"},
     *     mimeTypesMessage = "Please upload a valid image (JPEG or PNG) file"
     * )
     */
    private $file;

    // Getter and setter for $file
}

use Symfony\Component\HttpFoundation\Request;

public function yourAction(Request $request)
{
    $yourEntity = new YourEntity();
    $form = $this->createForm(YourFormType::class, $yourEntity);
    $form->handleRequest($request);

    if ($form->isSubmitted() && $form->isValid()) {
        // 处理表单数据
    }

    // 渲染表单视图
}