exchange character encoding from UTF-8 to SJIS, for IE or Excel users. ref: http://qiita.com/Oakbow/items/a62d1de9c62ee8fa2eb2

application_controller.rb

  # IEのダウンロードファイル名の文字化け対応
  def filename_for_download(filename)
    if request.env['HTTP_USER_AGENT'] =~ %r{MSIE} && request.env['HTTP_ACCEPT_LANGUAGE'] =~ /^ja/
      filename = Utf8ToWin31jCodeConverter.convert_to_windows31j(filename)
    end
    filename
  end

utf8_to_win31j_code_converter.rb

class Utf8ToWin31jCodeConverter

  class << self
    def convert_table
      [
        %w|301C FF5E|, # WAVE DASH            => FULLWIDTH TILDE
        %w|2212 FF0D|, # MINUS SIGN           => FULLWIDTH HYPHEN-MINUS
        %w|00A2 FFE0|, # CENT SIGN            => FULLWIDTH CENT SIGN
        %w|00A3 FFE1|, # POUND SIGN           => FULLWIDTH POUND SIGN
        %w|00AC FFE2|, # NOT SIGN             => FULLWIDTH NOT SIGN
        %w|2014 2015|, # EM DASH              => HORIZONTAL BAR
        %w|2016 2225|, # DOUBLE VERTICAL LINE => PARALLEL TO
      ]
    end

    def convert_to_windows31j(utf_8_str)
      utf_8_str.encode!("UTF-8-MAC", "UTF-8", :invalid => :replace, :undef => :replace, :replace => '')
      utf_8_str.encode!("UTF-8")

      utf_to_win31j_safe_str = Utf8ToWin31jCodeConverter.convert(utf_8_str)
      utf_to_win31j_safe_str.encode 'WINDOWS-31J'
    end

    def convert(str)
      result = str.dup
      convert_table.each do |arr|
        from   = code_point_to_char(arr[0])
        to     = code_point_to_char(arr[1])
        result = result.gsub(from, to)
      end
      result
    end

    def code_point_to_char(code_point)
      code_point.to_i(16).chr("UTF-8")
    end

    def char_to_code_point(char)
      char[0].unpack("U*").first.to_s(16).upcase
    end
  end
end