针对ajax请求，返回二进制流的形式，实现文件下载

        var options = {

          "url": "/haha/attachment/download.do?eventid=" + file.oid + "&token=" + localStorage.getItem("token"),

          "method": 'post'

        }

        var config = $.extend(true, {

          method: 'get'

        }, options);

        var $iframe = $('<iframe id="down-file-iframe" />');

        var $form = $('<form target="down-file-iframe" method="' + config.method + '" />');

        $form.attr('action', config.url);

        for (var key in config.data) {

          $form.append('<input type="hidden" name="' + key + '" value="' + config.data[key] + '" />');

        }

        $iframe.append($form);

        $(document.body).append($iframe);

        $form[0].submit();

        $iframe.remove();