Shared user's firewall blocking attachment viewing

I created a base and shared it with someone trying to access it on a bank computer. Understandably, the bank’s IT infrastructure is highly locked down and as part of that, presumably in their firewall set up, this user is unable to view attachments. She shared her screen yesterday with me and I can see that when clicking on a PDF attachment she can see it for a second but then gets a pop up saying the content is blocked.

She can otherwise access the base just fine, so it’s not that they have blocked altogether.
And when I have an attachment loaded the URL is still an URL, so it seems there must be something embedded in the PDF viewer code or attachment server that is triggering the block. She’s going to ask her IT people about this but does anyone know if there a URL unique to my base serving the attachments, such that she could ask them about whitelisting just that URL? I’m going to give her the base URL and and see if their firewall can whitelist PDFs from that URL.

You can see the url of an attachment by creating a new formula field that references the attachment field. Off the top of my head it is something like

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