-N %n.%t option on httrack commandline version
renames all downloaded files to *.html files.
This is new in v. 3.40!

The full command that I use here (for actually just downloading a bunch  of
files ($items) to a local folder $dest):

httrack -N %n.%t -Qb0c0qo0J80T30R3M5000000 $items -O $dest

Regards,
Frank


P.S. Actually -N %n.%t was already a work around for 
-g -N1003 which by the way produces exactly the 
same *.html bogus in v. 3.40.

... and I remember that -g used to produce some nagging extra files with some
sort of information that one had to remove by hand all the time. 

Since Httrack doesn't support multiple archive tags, I use it sometimes as a
threaded download machine for the bunch of files I need.