Suggestion:
Probably a good I idea to add this to the help/ manual systems as followings:
Attach this file to
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.
To do this you will need a two step process. First create the record, using a workflow, where the field is contained. Then associate the containing element (eg. the group containing the File Uploader) in the record and upload the file using the File Uploader.
Sometime like this