title: FCM Push Notification "content" entry
body: <p>I'm using FCM to publish push notifications to my users, which is working great so far. </p>
  
  <p>Currently I'm trying to implement a Notification Content Extension to deliver customized push notification and previews, which is working great with local notifications, following <a href="https://stackoverflow.com/a/38066583/3191334">this post</a>.
  As far I know, I have to set the <code>category</code> entry and my notification category identifier in the push notification, in order to tell iOS, which notification UI it is supposed to use. </p>
  
  <p>The problem is, when I send the following message to FCM, with the category entry set, FCM erases the entry or changes it to <code>gcm.notification.category</code>, depending where I place the category entry (<code>aps</code> / <code>data</code>, etc.)</p>
  
  <p>This way iOS never shows my custom UI / extension. Unfortunately I was not able to find any help in the <a href="https://firebase.google.com/docs/cloud-messaging/http-server-ref" rel="nofollow noreferrer">FCM documentation</a>.</p>
  
  <p>Send (POST: <a href="https://fcm.googleapis.com/fcm/send" rel="nofollow noreferrer">https://fcm.googleapis.com/fcm/send</a>): </p>
  
  <pre><code>{ 
      "notification": {
          "title": "Good Morning",
          "body": "Wake up Jack!",
          "badge" : 1,
          "sound" : "horn.aiff",
          "category" : "Cheers"     &lt;-- Is going to be deleted / changed
      },
  
      "data" : {
          "time" : "2018-01-19 23:00:00",
          ...
      },
  
      "mutable_content" : true,
  
      "priority" : "high",
      "registration_ids" : [
        "abcdefg123456"
      ]
  }
  </code></pre>
  
  <p>Received:</p>
  
  <pre><code>{
      aps =     {
          alert =         {
              body = "Wake up Jack!";
              title = "Good Morning";
          };
          badge = 1;
          "mutable-content" = 1;
      };
  
      "gcm.message_id" = "0:1516392279506894%dc84760ddc84760d";
      "gcm.notification.category" = "Cheers";    &lt;-- not working 
  }
  </code></pre>