> Is it possible that Httrack checks the mime-type in the
> http-response header before downloading (through Content
>-Type:) and allow / disallows the download for certain
> mime-types?
It will. 

It is, actually, in the current alpha releases:
See <http://www.httrack.com/html/filters.html>

Example (3.40 alpha only!)!:
-mime:*/* +mime:text/html +mime:image/*