The list of paths to trusted SSH key files controls the public / private key authentication that SFTP allows for. This setting does not want you to enter in a 'trusted keys file', or a folder path, but rather the path to the actual key file itself. So '/files/keys/' is not OK, while '/files/keys/ben.pub' would be OK. Separate multiple items with new lines. Most SSH key formats are supported.

There is also a more generic way to use this field. If the key file has the exact name of the user logging in, you can instead reference the directory '/files/keys/'. In this case there would need to be a file named 'ben' in that directory.