> I can understand why files with no extension get html or 
> txt extension when mirrored as web browser needs it 
etc.etc.
> But why is that when I mirror ftp site. I see no reason 
for 
> changing the file names in any way when mirroring ftp 
sites.

Ah, that's right. BUT consider this case:

<ftp://ftp.example.com/foobar>
<ftp://ftp.example.com/foobar/myfile.zip>

The first link will generate an index (the "ls" result from 
the ftp server)
The second one will generate a myfile.zip file

Unfortunately, filesystems do not allow the creation of 
both file and directory with same names:

ftp.example.com/foobar
^^^here, foobar would be a text file
ftp.example.com/foobar/myfile.zip
^^^here, foobar would be a directory

Okay, at this stage, you'll say "Renaming directory 
listings is okay, but renaming real files is unnecessary". 
The problem is that link build is done before downloading 
the file: httrack doesn't know if /foobar is a directory 
listing.. or a file.