module Sinatra::Helpers

def attachment(filename = nil, disposition = 'attachment')

instructing the user agents to prompt to save.
Set the Content-Disposition to "attachment" with the specified filename,

def attachment(filename = nil, disposition = 'attachment')
  response['Content-Disposition'] = disposition.to_s
  if filename
    params = '; filename="%s"' % File.basename(filename)
    response['Content-Disposition'] << params
    ext = File.extname(filename)
    content_type(ext) unless response['Content-Type'] or ext.empty?
  end
end