This option is only visible if 'Make this file private' is selected. Private files uploaded with this element are permanently attached to a thing, which is used to determine who has access to view the file. Only users who have the 'View attached files' permission for that thing can view the file. Go to the Privacy section to create privacy rules that grant this permission. If the value of 'Attach this file to' is empty or the thing does not exist, the file will be visible to anyone who has the link, which is the same as if 'Make this file private' was not selected.