File upload field public URL

Hi all,

I am on a multisite structure, and each time someone uploads a file via the file upload field, the file is available via public URL for every single site of the multisite structure.
How can I make the link “unavailable” or private someway? Or maybe it is enough to avoid the creation of the URL…

I am familiar with code, so I can even go through the plugin files to edit dome code strings.

Thanks for any help

https://wordpress.org/plugins/visual-form-builder/