Unfortunately, this is currently not implemented, so a private key authentication cannot be configured for a SFTP file listener. However, I've created an improvement proposal in our system. For the time being I cannot guarantee that it will be implemented but our developers will definitely take a look at it and decide if and when it will be implemented. You can watch the state of this issue here: https://bug.javlin.eu/browse/CLO-10299
I'm sorry for the inconvenience.