fixed MessageEmbed#timestamp and changed attachFile to attachFiles (#1670)

* fixed MessageEmbed#timestamp and changed attachFile to attachFiles
as well as a few doc changes

* Embed#files shouldn't be a nested array

src/structures/MessageEmbed.js

@@ -44,11 +44,11 @@ class MessageEmbed {
      * The timestamp of this embed
      * @type {?number}
      */
-    this.timestamp = new Date(data.timestamp);
+    this.timestamp = data.timestamp ? new Date(data.timestamp).getTime() : null;
 
     /**
      * The fields of this embed
-     * @type {?Object[]}
+     * @type {Object[]}
      * @property {string} name The name of this field
      * @property {string} value The value of this field
      * @property {boolean} inline If this field will be displayed inline
@@ -133,11 +133,11 @@ class MessageEmbed {
 
   /**
    * The date this embed was created
-   * @type {Date}
+   * @type {?Date}
    * @readonly
    */
   get createdAt() {
-    return !isNaN(this.timestamp) ? this.timestamp : null;
+    return this.timestamp ? new Date(this.timestamp) : null;
   }
 
   /**
@@ -178,12 +178,12 @@ class MessageEmbed {
   /**
    * Sets the file to upload alongside the embed. This file can be accessed via `attachment://fileName.extension` when
    * setting an embed image or author/footer icons. Only one file may be attached.
-   * @param {FileOptions|string} file Local path or URL to the file to attach, or valid FileOptions for a file to attach
+   * @param {Array<FileOptions|string>} files Files to attach
    * @returns {MessageEmbed} This embed
    */
-  attachFile(file) {
-    if (this.file) throw new RangeError('EMBED_FILE_LIMIT');
-    this.file = file;
+  attachFiles(files) {
+    if (this.files) this.files = this.files.concat(files);
+    else this.files = files;
     return this;
   }
 
@@ -295,7 +295,7 @@ class MessageEmbed {
       timestamp: this.timestamp,
       color: this.color,
       fields: this.fields,
-      file: this.file,
+      files: this.files,
       thumbnail: this.thumbnail,
       image: this.image,
       author: this.author ? {