Are you able to check the file in isolation? Can you make sure the file is UTF-8? How are you viewing the contents of the file i.e. are you sure the useragent used to view the contents is actually able to display these characters ?
Sorry... I also meant to say that the Ftp listener doesn't actually decode the file contents. It just reads the raw bytes into a byte and that's the message payload (see RemoteFileMessageComposer).