> WinHTTrack is copying css and js files to txt files
> Instead of files with the css, js or html file
> extensions I just see f1.txt, f2.txt and f3.txt
> 
> How can I correct this?
I don't try it myself, but in the help, I found:

  u  check document type if unknown (cgi,asp..) (u0 don't check, * u1 check
but /, u2 check always) (--check-type[=N])
 %A  assume that a type (cgi,asp..) is always linked with a mime type (-%A
php3,cgi=text/html;dat,bin=application/x-zip) (--assume <param>)
     shortcut: '--assume standard' is equivalent to -%A php2 php3 php4 php cgi
asp jsp pl cfm nsf=text/html
     can also be used to force a specific file type: --assume
foo.cgi=text/html

And additionally, the Guru option:
Guru options: (do NOT use if possible)
 #2  type test (-#2 /foo/bar.php)

You can try and let me know the results.