send_file_headers!

# File actionpack/lib/action_controller/streaming.rb, line 148
      def send_file_headers!(options)
        options.update(DEFAULT_SEND_FILE_OPTIONS.merge(options))
        [:length, :type, :disposition].each do |arg|
          raise ArgumentError, ":#{arg} option required" if options[arg].nil?
        end

        disposition = options[:disposition].dup || 'attachment'

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

        content_type = options[:type]
        if content_type.is_a?(Symbol)
          raise ArgumentError, "Unknown MIME type #{options[:type]}" unless Mime::EXTENSION_LOOKUP.has_key?(content_type.to_s)
          content_type = Mime::Type.lookup_by_extension(content_type.to_s)
        end
        content_type = content_type.to_s.strip # fixes a problem with extra '\r' with some browsers

        headers.merge!(
          'Content-Length'            => options[:length].to_s,
          'Content-Type'              => content_type,
          'Content-Disposition'       => disposition,
          'Content-Transfer-Encoding' => 'binary'
        )

        # 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
        headers['Cache-Control'] = 'private' if headers['Cache-Control'] == 'no-cache'
      end