send_file_headers!

# File actionpack/lib/action_controller/metal/data_streaming.rb, line 116
      def send_file_headers!(options)
        type_provided = options.has_key?(:type)

        content_type = options.fetch(:type, DEFAULT_SEND_FILE_TYPE)
        self.content_type = content_type
        response.sending_file = true

        raise ArgumentError, ":type option required" if content_type.nil?

        if content_type.is_a?(Symbol)
          extension = Mime[content_type]
          raise ArgumentError, "Unknown MIME type #{options[:type]}" unless extension
          self.content_type = extension
        else
          if !type_provided && options[:filename]
            # If type wasn't provided, try guessing from file extension.
            content_type = Mime::Type.lookup_by_extension(File.extname(options[:filename]).downcase.delete(".")) || content_type
          end
          self.content_type = content_type
        end

        disposition = options.fetch(:disposition, DEFAULT_SEND_FILE_DISPOSITION)
        if disposition
          headers["Content-Disposition"] = ActionDispatch::Http::ContentDisposition.format(disposition: disposition, filename: options[:filename])
        end

        headers["Content-Transfer-Encoding"] = "binary"
      end