download documents only filter thumbs when there are thumbs in url

main/document/download.php

@@ -83,8 +83,10 @@ if (Security::check_abs_path($sys_course_path.$doc_url, $sys_course_path.'/')) {
 	
 	//Documents' slideshow thumbnails
 	//correct $is_visible used in below and ??. Now the students can view the thumbnails too
-	$doc_url_thumbs = str_replace('.thumbs/.', '', $doc_url);
-	$is_visible = DocumentManager::is_visible($doc_url_thumbs, $_course, api_get_session_id());
+	if ( preg_match('/\.thumbs\/\./',  $doc_url)){ 
+		$doc_url_thumbs = str_replace('.thumbs/.', '', $doc_url);
+		$is_visible = DocumentManager::is_visible($doc_url_thumbs, $_course, api_get_session_id());
+	}
 	
     if (!api_is_allowed_to_edit() && !$is_visible) {
     	Display::display_error_message(get_lang('ProtectedDocument'));//api_not_allowed backbutton won't work.