gist.rb

if Rails.env.production?
  resp = HTTParty.get(@sale.product.file.url)

  filename = @sale.product.file.url
  send_data resp.body,
    :filename => File.basename(filename),
    :content_type => resp.headers['Content-Type']
else
  filename = File.expand_path(Sale.last.product.file_file_name, Rails.root)
  send_data File.open(@sale.product.file_file_name),
    :filename => File.basename(filename),
    :content_type => @sale.product.file_content_type
end