Added thumb to Audio entity.

src/Entities/Audio.php

@@ -21,8 +21,17 @@ namespace Longman\TelegramBot\Entities;
  * @method string    getTitle()     Optional. Title of the audio as defined by sender or by audio tags
  * @method string    getMimeType()  Optional. MIME type of the file as defined by sender
  * @method int       getFileSize()  Optional. File size
+ * @method PhotoSize getThumb()     Optional. Thumbnail of the album cover to which the music file belongs
  */
 class Audio extends Entity
 {
-
+    /**
+     * {@inheritdoc}
+     */
+    protected function subEntities()
+    {
+        return [
+            'thumb' => PhotoSize::class,
+        ];
+    }
 }