These are the options out of the box:
FTP even with login/passwd info: No really secure, data is send in clear text
FTP SSL/TSL: Somehow secure, safer than just FTP.
However, you might be able to get SCP or SFTP which are the most secured protocols for file transfers. For this, you will need fonz’s fun_plug. Never done it as I do not have my NAS open to the internet.