Yes that's what I was getting at. You cannot add fields to the File record. If there's a record that already gets created upon receipt of such a file, then you should be able to use those records to find them. If there's not already a field linking those records to the corresponding File, you could add it. If there's no record that gets created as a result, then you will likely need to create a Custom Record that has a File field and then any other data you'd like to track. AFAIK you can put Scripts and Workflows on the File record, so you could use either of those to populate the custom record whenever you receive a relevant File.