> site.url.com/Image.aspx?Mime=image/gif&Media=CD/CDF0401
> The image is set to .html (I setted off the mime types 

Humm, this means that httrack tries to issue HEAD requests 
to find the "real" type, and the server probably replied 
with "text/html"
Try to activate "use HTTP/1.0 requests" in httrack, this 
may work.

But it is also possible that the server is always sending 
binary contents with text/html type (this actually works 
with Internet Explorer - NOT with Mozilla or httrack)

> I compared that file with the original and I 
> get some &20 caracters where &00 was espected!!!

Yes, httrack was fooled by the "text/html" type sent by the 
server, most probably