haml by default

I use gem 'haml-rails', '= 0.3.4' in my gemfile. it automatically generates *.html.haml without any configuration.

In your application config, try setting the following:

config.generators do |g|
  g.template_engine :haml
end

The haml [not found] error is usually because the bundle is incomplete. Have you tried running bundle update and then rerunning the generator?