# File lib/action_mailer/vendor/tmail-1.2.3/tmail/interface.rb, line 941
    def set_disposition( str, params = nil )
      if h = @header['content-disposition']
        h.disposition = str
        h.params.clear
      else
        store('Content-Disposition', str)
        h = @header['content-disposition']
      end
      h.params.replace params if params
    end