I still have no idea why onclicks are disabled but I found a solution:
- just edit /wp-content/plugins/wp-filebase/classes/TplLib.php and comment line 15:
//$tpl = preg_replace(array('/<a\s+([^>]*)onclick=".+?"\s+([^>]*)href="%file_url%"/i', '/<a\s+([^>]*)href="%file_url%"\s+([^>]*)onclick=".+?"/i'), '<a href="%file_url%" $1$2', $tpl);