FCM rich push notification payload for iOS

The mutable-content and content-available in your FCM payload is incorrect. It should be formatted as mutable_content and content_available. Both are boolean and must also be outside the notification parameter. Like so:

{
   "to": "dWB537Nz1GA:APA91bHIjJ5....",
   "content_available": true,
   "mutable_content": true,
   "data":
   {
      "message": "Offer!",
      "mediaUrl": "https://upload.wikimedia.org/wikipedia/commons/thumb/2/2a/FloorGoban.JPG/1024px-FloorGoban.JPG"
   },
   "notification":
   {
      "body": "Enter your message",
      "sound": "default"
   }
}

For the counterpart of category in FCM, you should use click_action:

The action associated with a user click on the notification.

Corresponds to category in the APNs payload.

This worked for me. The accepted answer seems to have some unnecessary information.

{
  "to" : "devicekey OR /topics/sometopic",
  "mutable_content": true,
  "data": {
    "mymediavideo": "https://myserver.com/myvideo.mp4"
  },
  "notification": {
    "title": "my title",
    "subtitle": "my subtitle",
    "body": "some body"
  }
}