how to upload multiple file in a single form in codeigniter

[eluser]Unknown[/eluser]
can anybody help me out to upload multiple file from a single form. if possible please give some example. below is my code in model for upload function it works fine for single upload, but doesn't work for multiple uploads from a single form.

Code:

function upload(){
         $config[$a]=array(
            'allowed_types'=>'xlsx',
            'upload_path'=>  $this->gallery_path,
            'overwrite'=>true,
            'max_size'=>2000,
        );
         $this->load->library('upload',$config);
        $this->upload->do_upload();
        
    }

Thanks in Advance.