Not FTP - Secure FTP - SFTP.

When I was involved with this it was SFTP for the transfer, to a write-only folder (you couldn't list contents) and the file uploaded had to be PGP encrypted with a key the bank gave us and we verified over the phone before starting the process.

Even if it had been email, or plain FTP, the file was encrypted and only the bank had the key... even we couldn't decrypt it, due to the nature of Public Key encryption.

Plain old FTP is not considered secure by any means, and is largely disappearing - SFTP is standard now.