Add RichEmbed#attachFile (#1202)

* Add RichEmbed#attachFile

Mostly for attaching local images that can be accessed within the embed image/author icon/footer icon via `attachment//filename.png` and the like.

* Update docstring to reflect valid param types

* Update TextBasedChannel.js

* Update RichEmbed.js

* Update RichEmbed.js

src/structures/RichEmbed.js

@@ -65,6 +65,12 @@ class RichEmbed {
      * @type {Object}
      */
     this.footer = data.footer;
+
+    /**
+     * File to upload alongside this Embed
+     * @type {string}
+     */
+    this.file = data.file;
   }
 
   /**
@@ -191,6 +197,18 @@ class RichEmbed {
     this.footer = { text, icon_url: icon };
     return this;
   }
+
+  /**
+   * 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
+   * @returns {RichEmbed} This embed
+   */
+  attachFile(file) {
+    if (this.file) throw new RangeError('You may not upload more than one file at once.');
+    this.file = file;
+    return this;
+  }
 }
 
 module.exports = RichEmbed;

src/structures/interface/TextBasedChannel.js

@@ -78,6 +78,8 @@ class TextBasedChannel {
       options = {};
     }
 
+    if (options.embed && options.embed.file) options.file = options.embed.file;
+
     if (options.file) {
       if (typeof options.file === 'string') options.file = { attachment: options.file };
       if (!options.file.name) {