Add multifile support to webhooks

src/Discord.Net.Rest/API/Rest/UploadWebhookFileParams.cs

@@ -12,9 +12,8 @@ namespace Discord.API.Rest
     {
         private static JsonSerializer _serializer = new JsonSerializer { ContractResolver = new DiscordContractResolver() };
 
-        public Stream File { get; }
+        public IEnumerable<KeyValuePair<string, Stream>> Files { get; }
 
-        public Optional<string> Filename { get; set; }
         public Optional<string> Content { get; set; }
         public Optional<string> Nonce { get; set; }
         public Optional<bool> IsTTS { get; set; }
@@ -24,19 +23,23 @@ namespace Discord.API.Rest
 
         public bool IsSpoiler { get; set; } = false;
 
-        public UploadWebhookFileParams(Stream file)
+        public UploadWebhookFileParams(IEnumerable<KeyValuePair<string, Stream>> files)
         {
-            File = file;
+            Files = files;
         }
 
         public IReadOnlyDictionary<string, object> ToDictionary()
         {
             var d = new Dictionary<string, object>();
-            var filename = Filename.GetValueOrDefault("unknown.dat");
-            if (IsSpoiler && !filename.StartsWith(AttachmentExtensions.SpoilerPrefix))
-                filename = filename.Insert(0, AttachmentExtensions.SpoilerPrefix);
 
-            d["file"] = new MultipartFile(File, filename);
+            int i = 0;
+            foreach (var file in Files)
+            {
+                var filename = file.Key ?? "unknown.dat";
+                if (IsSpoiler && !filename.StartsWith(AttachmentExtensions.SpoilerPrefix))
+                    filename = filename.Insert(0, AttachmentExtensions.SpoilerPrefix);
+                d[$"file{i++}"] = new MultipartFile(file.Value, filename);
+            }
 
             var payload = new Dictionary<string, object>();
             if (Content.IsSpecified)

src/Discord.Net.Webhook/DiscordWebhookClient.cs

@@ -102,6 +102,17 @@ namespace Discord.Webhook
             IEnumerable<Embed> embeds = null, string username = null, string avatarUrl = null, RequestOptions options = null, bool isSpoiler = false)
             => WebhookClientHelper.SendFileAsync(this, stream, filename, text, isTTS, embeds, username, avatarUrl, options, isSpoiler);
 
+        /// <summary> Sends a message to the channel for this webhook with multiple attachments. </summary>
+        /// <returns> Returns the ID of the created message. </returns>
+        public Task<ulong> SendFileAsync(IEnumerable<string> filePaths, string text, bool isTTS = false,
+            IEnumerable<Embed> embeds = null, string username = null, string avatarUrl = null, RequestOptions options = null, bool isSpoiler = false)
+            => WebhookClientHelper.SendFileAsync(this, filePaths, text, isTTS, embeds, username, avatarUrl, options, isSpoiler);
+        /// <summary> Sends a message to the channel for this webhook with multiple attachments. </summary>
+        /// <returns> Returns the ID of the created message. </returns>
+        public Task<ulong> SendFileAsync(IEnumerable<KeyValuePair<string, Stream>> streams, string text, bool isTTS = false,
+            IEnumerable<Embed> embeds = null, string username = null, string avatarUrl = null, RequestOptions options = null, bool isSpoiler = false)
+            => WebhookClientHelper.SendFileAsync(this, streams, text, isTTS, embeds, username, avatarUrl, options, isSpoiler);
+
         /// <summary> Modifies the properties of this webhook. </summary>
         public Task ModifyWebhookAsync(Action<WebhookProperties> func, RequestOptions options = null)
             => Webhook.ModifyAsync(func, options);

src/Discord.Net.Webhook/WebhookClientHelper.cs

@@ -41,10 +41,24 @@ namespace Discord.Webhook
             using (var file = File.OpenRead(filePath))
                 return await SendFileAsync(client, file, filename, text, isTTS, embeds, username, avatarUrl, options, isSpoiler).ConfigureAwait(false);
         }
+        public static async Task<ulong> SendFileAsync(DiscordWebhookClient client, IEnumerable<string> filePaths, string text, bool isTTS,
+            IEnumerable<Embed> embeds, string username, string avatarUrl, RequestOptions options, bool isSpoiler)
+        {
+            var files = filePaths.Select(x => new KeyValuePair<string, Stream>(Path.GetFileName(x), File.OpenRead(x)));
+            var id = await SendFileAsync(client, files, text, isTTS, embeds, username, avatarUrl, options, isSpoiler).ConfigureAwait(false);
+            foreach (var file in files)
+                file.Value.Dispose();
+            return id;
+        }
         public static async Task<ulong> SendFileAsync(DiscordWebhookClient client, Stream stream, string filename, string text, bool isTTS,
             IEnumerable<Embed> embeds, string username, string avatarUrl, RequestOptions options, bool isSpoiler)
         {
-            var args = new UploadWebhookFileParams(stream) { Filename = filename, Content = text, IsTTS = isTTS, IsSpoiler = isSpoiler };
+            return await SendFileAsync(client, new[] { new KeyValuePair<string, Stream>(filename, stream) }, text, isTTS, embeds, username, avatarUrl, options, isSpoiler).ConfigureAwait(false);
+        }
+        public static async Task<ulong> SendFileAsync(DiscordWebhookClient client, IEnumerable<KeyValuePair<string, Stream>> files, string text, bool isTTS,
+            IEnumerable<Embed> embeds, string username, string avatarUrl, RequestOptions options, bool isSpoiler)
+        {
+            var args = new UploadWebhookFileParams(files) { Content = text, IsTTS = isTTS, IsSpoiler = isSpoiler };
             if (username != null)
                 args.Username = username;
             if (avatarUrl != null)