@peacemaker Why not using VPN Server, gate... to have access to your business-network and have user/password at the file server..?
To understand... you already have a web server and ftp, that upload/download their files (pdf, jpeg, doc) and users have their password to watch their folder ?
1) You can secure it with ssl certificate... also a client certificate for more security... add OTP auth
2) Buy a NAS Server hardware or software with ssl certifcate, usernames passwords, OTP auth - edit: some free software exist... hmmm... give a try to NextCloud (i think is still free and very good)
edit: 3) Google Cloud Drive... but need money...