send_file_headers!

# File actionpack/lib/action_controller/metal/streaming.rb, line 118
      def send_file_headers!(options)
        options.update(DEFAULT_SEND_FILE_OPTIONS.merge(options))
        [:type, :disposition].each do |arg|
          raise ArgumentError, ":#{arg} option required" if options[arg].nil?
        end

        if options.key?(:length)
          ActiveSupport::Deprecation.warn("You do not need to provide the file's length", caller)
        end

        disposition = options[:disposition]
        disposition += %(; filename="#{options[:filename]}") if options[:filename]

        content_type = options[:type]

        if content_type.is_a?(Symbol)
          extension = Mime[content_type]
          raise ArgumentError, "Unknown MIME type #{options[:type]}" unless extension
          self.content_type = extension
        else
          self.content_type = content_type
        end

        headers.merge!(
          'Content-Disposition'       => disposition,
          'Content-Transfer-Encoding' => 'binary'
        )

        response.sending_file = true

        # Fix a problem with IE 6.0 on opening downloaded files:
        # If Cache-Control: no-cache is set (which Rails does by default),
        # IE removes the file it just downloaded from its cache immediately
        # after it displays the "open/save" dialog, which means that if you
        # hit "open" the file isn't there anymore when the application that
        # is called for handling the download is run, so let's workaround that
        response.cache_control[:public] ||= false
      end