saving file to specific location

Question

here is my method to saving file:

 def savefile
    @generator = Generator.new(params[:generator])
    @bigtable = Rails.cache.read("pass")
    doc = "wyniki.csv"
    File.open(doc, "w"){ |f| f << @bigtable}
    send_file(doc, :type => 'text; charset=utf-8')
  end

Currently, my app is saving my file to default downloading browser location - I want to change it - I want initialize pop-up window where user can change filename and location where it should be saved (default downloading browser window)- can anyone help me how to do this?