title: Logstash rename field with fileset.name
body: <p>I am using Logstash 6.2.1, which collects the syslogs from Filebeat. I am receiving two fields (as seen in Kibana), <code>fileset.name</code> and <code>fileset.module</code> with the values <em>syslog</em> and <em>system</em> respectively. I wish to rename these fields in Logstash with the following filter </p>
  
  <pre><code>filter {
     mutate {
       rename =&gt; { "fileset.module" =&gt; "category"}
     }
  }
  </code></pre>
  
  <p>but the field names does not change. I can change some of the other fields, it appears those with no fullstops in their names.</p>