Popup stopped appearing in app

When a user wants to upload a file to our community website from Safari, they select Upload and they should get a popup that shows the files on their local Mac. They can then select the file to upload. I have a user where this stopped working and the popup doesn't appear any longer so they can't upload files. I'm guessing there is something or a setting that is blocked. We checked Safari Preferences and nothing stands out that should be corrected. Any suggestions on what to check? Any settings that might cause this and how to unblock?