format_from_path_extension

# File actionpack/lib/action_dispatch/http/mime_negotiation.rb, line 184
        def format_from_path_extension
          path = get_header("action_dispatch.original_path") || get_header("PATH_INFO")
          if match = path && path.match(/\.(\w+)\z/)
            Mime[match.captures.first]
          end
        end