     case NoDownload     = 0 // 未下载
     case Downloading    = 1 // 下载中
     case Downloaded     = 2 // 已下载
+}
+
+enum FileExtension: String {
+    case Image = "jpg"
+    case Video = "mp4"
+    case Audio = "m4a"
 }
 
 enum MessageMediaType: Int, CustomStringConvertible {

    case Video = "video"
}


class Attachment: Object {

    dynamic var kind: String = ""

        }
    }

    var fileExtension: FileExtension? {
        switch self {
        case .Image:
            return .Image
        case .Video:
            return .Video
        case .Audio:
            return .Audio
        default:
            return nil
        }
    }

    var placeholder: String? {
        switch self {
        case .Text:
