Fix for sending files with Webhook class (discordjs#1449)

src/structures/Webhook.js

@@ -81,14 +81,15 @@ class Webhook {
    * @property {boolean} [disableEveryone=this.client.options.disableEveryone] Whether or not @everyone and @here
    * should be replaced with plain-text
    * @property {FileOptions|string} [file] A file to send with the message
+   * @property {FileOptions[]|string[]} [files] Files to send with the message
    * @property {string|boolean} [code] Language for optional codeblock formatting to apply
    * @property {boolean|SplitOptions} [split=false] Whether or not the message should be split into multiple messages if
    * it exceeds the character limit. If an object is provided, these are the options for splitting the message.
    */
 
   /**
    * Send a message with this webhook.
-   * @param {StringResolvable} content The content to send
+   * @param {StringResolvable} [content] The content to send
    * @param {WebhookMessageOptions} [options={}] The options to provide
    * @returns {Promise<Message|Message[]>}
    * @example
@@ -104,24 +105,36 @@ class Webhook {
     } else if (!options) {
       options = {};
     }
+
     if (options.file) {
-      if (typeof options.file === 'string') options.file = { attachment: options.file };
-      if (!options.file.name) {
-        if (typeof options.file.attachment === 'string') {
-          options.file.name = path.basename(options.file.attachment);
-        } else if (options.file.attachment && options.file.attachment.path) {
-          options.file.name = path.basename(options.file.attachment.path);
-        } else {
-          options.file.name = 'file.jpg';
+      if (options.files) options.files.push(options.file);
+      else options.files = [options.file];
+    }
+
+    if (options.files) {
+      for (const i in options.files) {
+        let file = options.files[i];
+        if (typeof file === 'string') file = { attachment: file };
+        if (!file.name) {
+          if (typeof file.attachment === 'string') {
+            file.name = path.basename(file.attachment);
+          } else if (file.attachment && file.attachment.path) {
+            file.name = path.basename(file.attachment.path);
+          } else {
+            file.name = 'file.jpg';
+          }
         }
+        options.files[i] = file;
       }
-      return this.client.resolver.resolveBuffer(options.file.attachment).then(file =>
-        this.client.rest.methods.sendWebhookMessage(this, content, options, {
-          file,
-          name: options.file.name,
+
+      return Promise.all(options.files.map(file =>
+        this.client.resolver.resolveBuffer(file.attachment).then(buffer => {
+          file.file = buffer;
+          return file;
         })
-      );
+      )).then(files => this.client.rest.methods.sendWebhookMessage(this, content, options, files));
     }
+
     return this.client.rest.methods.sendWebhookMessage(this, content, options);
   }