string 转 number问题

怎样才能把收上来日志字段转换string为number? 我已经再logstash里配置转换为integer了，但是到kibana里显示还是string, 还没法转换。如下图：


 logstash conf:
convert => { "upload" => "integer" }
convert => { "download" => "integer" } 
 
logstash 输出：
{
       "message" => "Apr 18 11:14:12 localhost sftp-server[4022]: close \"/etc/logstash/conf.d/logstash_out.conf\" bytes read 0 written 1745",
          "tags" => [
        [0] "l_sftp"
    ],
        "source" => "/var/log/sftp.log",
    "@timestamp" => 2018-04-18T03:14:14.876Z,
      "filename" => "/etc/logstash/conf.d/logstash_out.conf",
      "hostname" => "elk",
      "download" => "0",
        "upload" => "1745"
}