AJAX multiple upload using FormData()

I'd like to do multiple upload using formData() and AJAX. My AJAX successfully sent data to controller. This is my script:

$(".upl_user_document").each(function(i){
    regist_data.append('bill_doc-'+i, $(this)[0].files[0]);
});
                    
regist_data.append("data_ci",JSON.stringify(data_form));
regist_data.append("data_epicor",JSON.stringify(JSON_regist));
regist_data.append("<?php echo $this->security->get_csrf_token_name(); ?>",csrfHash);
                
    $.ajax({
        url         : url,                      
        type        : 'POST',                       
        enctype     : 'multipart/form-data',                        
        dataType    : 'JSON',
        contentType : false,
        processData : false,
        cache       : false,
        data        : regist_data
    })
    .done(function(data){
        
        //SUCCESS                           
    })
    .fail(function(jqXHR, textStatus, errorThrown){
         
        //FAIL              
    });

This script success sent data to the controller. But, I got undefined index bill_doc error in my controller. This is my controller:

//UPLOAD DOC                    
$my_upload  = $_FILES['bill_doc'];
$j_upl      = count($my_upload["name"]);    
$files      = $_FILES;

for($i=0;$i<$j_upl;$i++){                       

    if(isset($_FILES) & !empty($_FILES)){
        $name     = $files['file']['name'];                      
        $tmp_name = $files['file']['tmp_name']; 
    }

    if(!file_exists(FCPATH."asset/asset_form_signup/uploads/".$epicor_id)) {

        mkdir(FCPATH."asset/asset_form_signup/uploads/".$epicor_id, 0755, true);
    }

    $config['upload_path']   = FCPATH."asset/asset_form_signup/uploads/".$epicor_id."/";

    $this->load->library('upload', $config);

    $_FILES['bill_doc']['name']     = $files['bill_doc']['name'][$i];
    $_FILES['bill_doc']['type']     = $files['bill_doc']['type'][$i];
    $_FILES['bill_doc']['tmp_name'] = $files['bill_doc']['tmp_name'][$i];
    $_FILES['bill_doc']['error']    = $files['bill_doc']['error'][$i];
    $_FILES['bill_doc']['size']     = $files['bill_doc']['size'][$i];

    $this->upload->initialize($config);

    $this->upload->do_upload('bill_doc');

    }

    $response_array = array ( "insert_message"  => "SUCCESS",
                              "token"           => $new_token);

    echo json_encode($response_array);
}

Do I do some mistake? The AJAX show no error. Any helpis greatly appreciated