Add missing jsonproperty attribute

src/Discord.Net.Rest/API/Common/Attachment.cs

@@ -18,6 +18,7 @@ namespace Discord.API
         public Optional<int> Height { get; set; }
         [JsonProperty("width")]
         public Optional<int> Width { get; set; }
+        [JsonProperty("ephemeral")]
         public Optional<bool> Ephemeral { get; set; }
     }
 }