    # 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