PHP/Symfony/Twig forms and filtering for URL entries

Question

I'm doing a school project in which I make a PHP/Symfony app. There is a form with a field which requires the user to enter an URL for an image hosted elsewhere. The problem is that when submitting the form, my hosting solution's firewall detects a 'Remote File Injection Attack'.

I tried to 'sanitize' the entry with a Twig filter (url_encode) however this does not seem to worl out (I'm quite the beginner as you may have realized by now)

I tried putting the filter after the 'row' :

{{ form_row(formformation.miniature)|url_encode() }}

... which encodes the entire field :

Then i tried putting it inside the row :

{{ form_row(formformation.miniature|url_encode()) }}

... which gives an error as the parameter needs to be a string, not an object.

The field in question is a simple text type field (I tried UrlType, without success)

Extract from the form :

                ->add('miniature', TextType::class, [
                    'label' =>'Miniature (URL, taille 120x90 pixels)',
                    'required'=>false
                ])

Extract from the entity :

     * @Assert\Length(max=46,maxMessage = "L'url de la miniature ne peut pas excéder {{ limit }} caractères")
     * @ORM\Column(type="string", length=46, nullable=true)
     */
    private $miniature;

I tried other filters including 'raw', enabling/disabling autoescape, nothing seems to work. I'm probably doing something very stupid :) Any ideas would be very much appreciated! Many thanks, Carl