Why file.js.coffee.erb is not working with respond_to?

I would like to know why a coffee-script file is not working when it’s named as file.js.coffee.erb and used with respond_to.

This is an example:

my_controller.rb:

def create
respond_to do |format|
format.html { render :new }
format.js { render :new }
end
end

The file in this case is named new.js.coffee.erb

When the create route is called with Ajax, the HTML version (new.html.erb) is always returned. If I rename the js file as new.js.coffee then it works correctly.

Why can’t the .erb suffix be added?