将现有文件粘贴到编辑表单中

When I create a new product, I load the required image and everything is fine.

But when I open the page with the form for editing, again ask to upload the file, because the field with the picture is mandatory, how do I insert an existing file into the form or how to get around this point?

Product:

class Product {
    /**
     * @var string
     * @Assert\NotBlank
     */
    private $image;
    // ...
}

ProductType:

class ProductType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
           // ...
           ->add('image', FileType::class, [
                'data_class' => null
            ])
        ;
    }
   // ...
}

As @JimPanse told you, you should read the documentation https://symfony.com/doc/current/controller/upload_file.html which explain you clearly how to manager file upload. Display the file is not the default behavior

After that, you should add a line into your twig to display your image

form.vars.value.image will return the value of your field image

Add this to your twig

{% if form.vars.value.image is defined and form.vars.value.image is not null %}
    <img alt="product" src="{{ form.vars.value.image }}"/>
{% endif %}

Or another way is to override the FileType template