Thumbnails are displayed

The bug with the get methode was fixed. Unnecessary code were deleted. The function  "if (has_capability('moodle/course:update',$context)) in comments.php was deleted because I think that the students should be write comments. I added the string media_notauth in lang/en/slideshow.php and in media.php.

The Thumbnails are displayed correctly on the caption page, comments page and the media page. Various file extensiones are accepted.