module Sinatra::Helpers

def attachment(filename=nil)

instructing the user agents to prompt to save.
Set the Content-Disposition to "attachment" with the specified filename, 
def attachment(filename=nil)
  response['Content-Disposition'] = 'attachment'
  if filename
    params = '; filename="%s"' % File.basename(filename)
    response['Content-Disposition'] << params
  end
end