Maybe you could check it with option:

%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>)

using this MIME type: "application/visio", e.g.:

httrack www.domain.com -%A vss=application/visio

Haven't tried it, but I assume that way they will download as binaries,
without changing their file extension.

Hope this helps