MDL-28562 "automatic naming of file in convert_image not working"

there is an obvious typo in the code causing the problem. The use of
an equality check instead of the intended assignment.

lib/filestorage/file_storage.php

@@ -994,7 +994,7 @@ class file_storage {
         }
 
         if (!isset($file_record['filename'])) {
-            $file_record['filename'] == $file->get_filename();
+            $file_record['filename'] = $file->get_filename();
         }
 
         if (!isset($file_record['mimetype'])) {