This is unfortunate, and, in my eyes a major limitation.
HTTrack can't make an exact mirror of a some websites.
I have such a website that I want to mirror. 
Most of the pages have no extension.
I can set apache up to default to text/html so the mirror can be served up
just fine.
However, I cannot use HTTrack to make the mirror due to this limitation.
I am looking for another product :-(