animated guild banner support (#255)

src/Discord.Net.Core/CDN.cs

@@ -139,15 +139,17 @@ namespace Discord
         /// </summary>
         /// <param name="guildId">The guild snowflake identifier.</param>
         /// <param name="bannerId">The banner image identifier.</param>
+        /// <param name="format">The format to return.</param>
         /// <param name="size">The size of the image to return in horizontal pixels. This can be any power of two between 16 and 2048 inclusive.</param>
         /// <returns>
         ///     A URL pointing to the guild's banner image.
         /// </returns>
-        public static string GetGuildBannerUrl(ulong guildId, string bannerId, ushort? size = null)
+        public static string GetGuildBannerUrl(ulong guildId, string bannerId, ImageFormat format, ushort? size = null)
         {
-            if (!string.IsNullOrEmpty(bannerId))
-                return $"{DiscordConfig.CDNUrl}banners/{guildId}/{bannerId}.jpg" + (size.HasValue ? $"?size={size}" : string.Empty);
-            return null;
+	        if (string.IsNullOrEmpty(bannerId))
+		        return null;
+	        string extension = FormatToExtension(format, bannerId);
+	        return $"{DiscordConfig.CDNUrl}banners/{guildId}/{bannerId}.{extension}" + (size.HasValue ? $"?size={size}" : string.Empty);
         }
         /// <summary>
         ///     Returns an emoji URL.

src/Discord.Net.Rest/Entities/Guilds/RestGuild.cs

@@ -102,7 +102,7 @@ namespace Discord.Rest
         /// <inheritdoc />
         public string DiscoverySplashUrl => CDN.GetGuildDiscoverySplashUrl(Id, DiscoverySplashId);
         /// <inheritdoc />
-        public string BannerUrl => CDN.GetGuildBannerUrl(Id, BannerId);
+        public string BannerUrl => CDN.GetGuildBannerUrl(Id, BannerId, ImageFormat.Auto);
 
         /// <summary>
         ///     Gets the built-in role containing all users in this guild.

src/Discord.Net.WebSocket/Entities/Guilds/SocketGuild.cs

@@ -138,7 +138,7 @@ namespace Discord.WebSocket
         /// <inheritdoc />
         public string DiscoverySplashUrl => CDN.GetGuildDiscoverySplashUrl(Id, DiscoverySplashId);
         /// <inheritdoc />
-        public string BannerUrl => CDN.GetGuildBannerUrl(Id, BannerId);
+        public string BannerUrl => CDN.GetGuildBannerUrl(Id, BannerId, ImageFormat.Auto);
         /// <summary> Indicates whether the client has all the members downloaded to the local guild cache. </summary>
         public bool HasAllMembers => MemberCount <= DownloadedMemberCount;// _downloaderPromise.Task.IsCompleted;
         /// <summary> Indicates whether the guild cache is synced to this guild. </summary>