Fix a glitch in thumb creation, a=chris

diff --git a/src/models/GroupModel.php b/src/models/GroupModel.php
index d5466f0e8..feba9b745 100644
--- a/src/models/GroupModel.php
+++ b/src/models/GroupModel.php
@@ -1449,7 +1449,7 @@ class GroupModel extends Model implements MediaConstants
                 $thumb_string);
             clearstatcache("$thumb_folder/$file_name.jpg");
         }
-        if (defined('FFMPEG') && in_array($mime_type, [
+        if (C\nsdefined('FFMPEG') && in_array($mime_type, [
             'video/mp4', 'video/webm', 'video/ogg', 'video/avi',
             'video/quicktime'])) {
             $make_thumb_string =