{
    "from": "Guille",
    "text": "Look what I just found!",
    "attachments": [
        {
            "type": "image",
            "payload": {
                "url": "http://via.placeholder.com/640x480",
                "width": 640,
                "height": 480
            }
        },
        {
            "type": "audio",
            "payload": {
                "title": "Never Gonna Give You Up",
                "url": "https://audio.com/NeverGonnaGiveYouUp.mp3",
                "shouldAutoplay": true,
            }
        }
    ]
}

struct ImageAttachment: Codable {
    let url: URL
    let width: Int
    let height: Int
}
struct AudioAttachment: Codable {
    let title: String
    let url: URL
    let shouldAutoplay: Bool
}

enum Attachment {
  case image(ImageAttachment)
  case audio(AudioAttachment)
  case unsupported
}

extension Attachment: Codable {
  private enum CodingKeys: String, CodingKey {
    case type
    case payload
  }
  init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    let type = try container.decode(String.self, forKey: .type)
    switch type {
    case "image":
      let payload = try container.decode(ImageAttachment.self, forKey: .payload)
      self = .image(payload)
    case "audio":
      let payload = try container.decode(AudioAttachment.self, forKey: .payload)
      self = .audio(payload)
    default:
      self = .unsupported
    }
  }
  ...
}