Add isSpoiler support to webhook client

adds the isSpoiler field to uploading files with a webhook, which will only
insert "SPOILER_" to the start of the filename. This does not include other
fields in the payload, as this is not in the documentation, and was not observed
like in the regular client

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

@@ -22,6 +22,8 @@ namespace Discord.API.Rest
         public Optional<string> AvatarUrl { get; set; }
         public Optional<Embed[]> Embeds { get; set; }
 
+        public bool IsSpoiler { get; set; } = false;
+
         public UploadWebhookFileParams(Stream file)
         {
             File = file;
@@ -30,7 +32,11 @@ namespace Discord.API.Rest
         public IReadOnlyDictionary<string, object> ToDictionary()
         {
             var d = new Dictionary<string, object>();
-            d["file"] = new MultipartFile(File, Filename.GetValueOrDefault("unknown.dat"));
+            var filename = Filename.GetValueOrDefault("unknown.dat");
+            if (IsSpoiler && !filename.StartsWith(AttachmentExtensions.SpoilerPrefix))
+                filename = filename.Insert(0, AttachmentExtensions.SpoilerPrefix);
+
+            d["file"] = new MultipartFile(File, filename);
 
             var payload = new Dictionary<string, object>();
             if (Content.IsSpecified)

src/Discord.Net.Webhook/DiscordWebhookClient.cs

@@ -71,13 +71,13 @@ namespace Discord.Webhook
         /// <summary> Sends a message to the channel for this webhook with an attachment. </summary>
         /// <returns> Returns the ID of the created message. </returns>
         public Task<ulong> SendFileAsync(string filePath, string text, bool isTTS = false,
-            IEnumerable<Embed> embeds = null, string username = null, string avatarUrl = null, RequestOptions options = null)
-            => WebhookClientHelper.SendFileAsync(this, filePath, text, isTTS, embeds, username, avatarUrl, options);
+            IEnumerable<Embed> embeds = null, string username = null, string avatarUrl = null, RequestOptions options = null, bool isSpoiler = false)
+            => WebhookClientHelper.SendFileAsync(this, filePath, text, isTTS, embeds, username, avatarUrl, options, isSpoiler);
         /// <summary> Sends a message to the channel for this webhook with an attachment. </summary>
         /// <returns> Returns the ID of the created message. </returns>
         public Task<ulong> SendFileAsync(Stream stream, string filename, string text, bool isTTS = false,
-            IEnumerable<Embed> embeds = null, string username = null, string avatarUrl = null, RequestOptions options = null)
-            => WebhookClientHelper.SendFileAsync(this, stream, filename, text, isTTS, embeds, username, avatarUrl, options);
+            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> Modifies the properties of this webhook. </summary>
         public Task ModifyWebhookAsync(Action<WebhookProperties> func, RequestOptions options = null)

src/Discord.Net.Webhook/WebhookClientHelper.cs

@@ -35,16 +35,16 @@ namespace Discord.Webhook
             return model.Id;
         }
         public static async Task<ulong> SendFileAsync(DiscordWebhookClient client, string filePath, string text, bool isTTS, 
-            IEnumerable<Embed> embeds, string username, string avatarUrl, RequestOptions options)
+            IEnumerable<Embed> embeds, string username, string avatarUrl, RequestOptions options, bool isSpoiler)
         {
             string filename = Path.GetFileName(filePath);
             using (var file = File.OpenRead(filePath))
-                return await SendFileAsync(client, file, filename, text, isTTS, embeds, username, avatarUrl, options).ConfigureAwait(false);
+                return await SendFileAsync(client, file, filename, text, isTTS, embeds, username, avatarUrl, options, isSpoiler).ConfigureAwait(false);
         }
         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)
+            IEnumerable<Embed> embeds, string username, string avatarUrl, RequestOptions options, bool isSpoiler)
         {
-            var args = new UploadWebhookFileParams(stream) { Filename = filename, Content = text, IsTTS = isTTS };
+            var args = new UploadWebhookFileParams(stream) { Filename = filename, Content = text, IsTTS = isTTS, IsSpoiler = isSpoiler };
             if (username != null)
                 args.Username = username;
             if (avatarUrl != null)