I am afraid this isn’t currently an option on the FTP node, so for now I shall convert your question into a feature request. Make sure to leave a vote on it so this can be considered in future versions of n8n.
Perhaps there are alternative approaches you can try in the meantime? For example, if your SFTP server allows ssh access, would you be able to use the ls command in the SSH Node instead?
This thread itself is the feature request. In the upper left you should see a Vote button (it says “Limit” for me as I have already used my available votes):