Having problem with my language file

The %s replacement you want to use is part of the sprintf PHP function. You could do something like this in your controller:

PHP Code:

public function multi_upload($field = 'userfile') {

    $this->load->helper('language');
    $this->lang->load('upload', 'english');

    ...

    if (!empty($this->files[$field]['name'][0])) {

        foreach ($this->files[$field]['name'] as $key => $value) {

            $get_file_extension     = explode('.', $value);
            $get_file_extension_end = strtolower(end($get_file_extension));
            $allowed_extension      = explode('|', $this->allowed_types);

            if (!in_array($get_file_extension_end, $allowed_extension)) {
                $errormsg = sprintf(lang('file_extension_not_allowed'), $get_file_extension_end)
                $this->set_error($errormsg);
                return FALSE;
            }
        }
    }
} 


And in your language file:

PHP Code:

$lang['file_extension_not_allowed'] =  'Your %s file is not allowed!';