If you came across dialog.filepicker.io and wonder if it's legitimate, you'll find that there's hardly anything to evaluate. The subdomain, part of the well-known Filepicker service, currently serves only a 404 error page. No contact info, no privacy policy, no about page — but those are irrelevant because there's no actual website here.
Is dialog.filepicker.io a scam? The signals don't suggest anything malicious: no blacklists, a clean Google Safe Browsing report, and a valid SSL certificate. The more interesting quirk is that the site's robots.txt blocks all crawlers, which is unusual for a legitimate business but makes sense if this is a leftover endpoint no one maintains.
Dialog.filepicker.io reviews are nonexistent because there's nothing to review. For the typical user, this subdomain is harmless but worthless. The only real risk is wasting a few seconds on a broken link.