Pointing the "Attach Files" Function of Expense Reports

Our expense report line items didn't previously have the ability to attach files. I've gone and turned this functionality on, but when I select list to show the available files, the list shows every file in the system. .js suitescripts, random pdfs, and on and on. Additionally, if I try to import a file, the available folders to store that file are an equally enormous and disorganized list.

Ideally I would like the "list" of files to point to an employee-specific expense folder, and for that folder to also be the only available one to store new files. How can I limit the scope of these options to such a folder? Thanks!