Sending a file message

{
   "to":"441234567890",
   "message":{
      "type":"file",
      "thumbnail":{
         "file_uri":"https://mydomain.net/path/to/thumbnail.jpg"
      },
      "file":{
         "file_name":"image.jpg",
         "file_uri":"https://mydomain.net/path/to/image.jpg"
      }
   }
}