title: Logstash dissect filter is mapping to undefined type
body: <p>I am using Filebeat to send a log to Logstash, then to Elasticsearch and to Kibana.
  Because each line of the log contains values delimited by '###', I use the 'dissect' filter to split up a single message block into different fields. My filter configuration:</p>
  
  <pre><code>dissect {
    mapping =&gt; {
      "message" =&gt; "%{date} %{time}###%{mode}###%{spec}###%{userid}### ..."
    }
    remove_field =&gt; [ "message" ]
  }
  </code></pre>
  
  <p>So i dissect and remove the original message object. It works but it maps it to keywords of type 'undefined'
  As shown in the screenshot below, it puts a '?' next to the field in Kibana.</p>
  
  <p><a href="https://i.stack.imgur.com/uSXjD.png" rel="nofollow noreferrer">Kibana screenshot</a></p>
  
  <p>How can I make sure that it dissects to type 'string' or 'text'? Right now it is impossible to visualize this data, because it does not work with 'undefined' types.</p>