How to FTP to and from a server that is accessible only via authentication-key pair

Scenario
Your backend is accessible only through ssh key-pairing but you have the need to use an FTP program like FileZilla to transfer a file from or to a local machine.

Temporarily disable the ssh only access
From the command line:

1. sudo nano /etc/ssh/sshd_config
2. Set PasswordAuthentication to yes
3. sudo service sshd restart

From your FTP application

1. Host sftp://your.ip.add.ress
2. Supply your username (do not use the root) and password
3. Get connected

When done with your task, revert the PasswordAuthentication to no.