feat(MessageAttachment): description (alt text) support (#6871)

Co-authored-by: D Trombett <maxtromb.dt@gmail.com>

src/structures/MessageAttachment.js

@@ -21,6 +21,16 @@ class MessageAttachment {
     if (data) this._patch(data);
   }
 
+  /**
+   * Sets the description of this attachment.
+   * @param {string} description The description of the file
+   * @returns {MessageAttachment} This attachment
+   */
+  setDescription(description) {
+    this.description = description;
+    return this;
+  }
+
   /**
    * Sets the file of this attachment.
    * @param {BufferResolvable|Stream} attachment The file
@@ -122,6 +132,16 @@ class MessageAttachment {
       this.contentType ??= null;
     }
 
+    if ('description' in data) {
+      /**
+       * The description (alt text) of this attachment
+       * @type {?string}
+       */
+      this.description = data.description;
+    } else {
+      this.description ??= null;
+    }
+
     /**
      * Whether this attachment is ephemeral
      * @type {boolean}