Historically we use the HTML extension for static exports because some browsers get confused viewing HTML files without an extension from a file:// URI.
You can change things by taking a copy of the template $:/core/templates/static.tiddler.html and changing the first line from:
\define tv-wikilink-template() $uri_doubleencoded$.html
to:
\define tv-wikilink-template() $uri_doubleencoded$
To render tiddlers using that template, you’d use a command like this, substituting the name of your modified template for the last argument.
--render "[!is[system]]" "[encodeuricomponent[]]" "text/html" "$:/core/templates/static.tiddler.html”