How to handle checkboxes

Inside your controller you can run a loop for every checked checkbox and attach a logical named file for each value like (untested)

PHP Code:

foreach($this->input->post('attachments') as $attachement)
{
    // $attachement contains for e.g. nui, lidar, etc.
    $this->email->attach('/attachments/file_'.$attachement.'.pdf');
}