The FTP protocol is officially defined to use the telnet protocol
as transport channel for the control connection. However, many
implementations are broken and does not respect this aspect of
the FTP protocol.
If you have trouble accessing files with ASCII code 255 in the
path or similar problems involving this ASCII code you can
try setting this directive to off. If that helps, report to the
operator of the FTP server in question that their FTP server
is broken and does not follow the FTP standard.