I was going to mirror some stuff from:

<ftp://www.rtsi.com/OS9/OS9_6X09/>

(and old 8 bit micro archive site) and all winhttrack 3.33 did was download
index.txt file.

There's not even an index.txt file there.  So apparently it misunderstands the
directory listing as being a plain text file.

Internet explorer doesn't have the problem.

The site is very slow, but I doubt that's the problem.