Having problem with my language file

'%s' means a string format, a '%d' can only replace a digit. In your case you want to replace 2 strings so you should use 2 '%s' signs.

PHP Code:

$lang['file_extension_not_allowed'] =  'Your file %s with the extension %s is not allowed!'; 
...
$line = sprintf($msg, $this->file_name, $this->get_file_extension_end);