How to apply filters to external sites? External non-html resourses are needed,
but only of a specific type. Example:

httrack <http://site> '-* +*.html*[] +*.css +*.png' -n

If 'site' contains non-html links to 'anothersite', all these links are
grabbed without respect to the filter rules. For instance, I need only
graphical non-html external resourses, but not gzip- or pdf-ones. Name of
'anothersite' is not known beforehand (and there may be several external
sites), so I can not include it in the filter rules. If option -n is omitted,
exernal links are not followed at all. Is there a decision of this problem?