Why a Galley available at a separate website with an URL is not considered as supplementary file?


We are using OJS 3.1.2.

We would like to add to the article page a galley available at a separate website by indicating the URL of remotely-hosted content.
But when we do that by adding a new galley, we can’t choose as for the other galleys to select an article component and the galley is automatically associated as a “primary galley” and not as a “supplementary file”.
Why is it not possible to add a galley with an URL and associated it as a supplementary file ?

Thanks in advance for your answer.
Best regards.