Well, you could run FTP, but if security is paramount (and I suppose it should be), then Secure FTP (SFTP) would work well, too, as it encrypts its communications. If your outside clients use Windows, then WinSCP is a good SFTP application for that platform to access your fileserver securely.
The nice thing about an SFTP solution is you get a similar experience to FTP, but it's secure, and without a major investment in time installing/configuring/managing anything new like a VPN solution (even if it already does come with OpenBSD.) Besides, a VPN solution suggests more than just needing access to files on a file server, which is all you have stated is needed to be happy.
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Network Firefighter
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