How to use Uploaded Files Library

I want upload some files in a Codeigniter4 program

The method in Controller:

Code:

<?php
namespace App\Controllers;
use CodeIgniter\Controller;


class Test extends Controller
{
    ……

    public function get(){
        $file = $this->request->getFile('thumbnail');
        dd($file->getName());
    }
    ……

Result(ci.com/test/get):

Error
Call to a member function getName() on null

How to use Uploaded Files Library?

Thanks!

This problem has been solved：
The upload file form must contain enctype="multipart/form-data"

Like this：

Code:

<form action="upload_file.php" method="post" enctype="multipart/form-data">

First Register service

Code:

$file = new \CodeIgniter\Files\File($path);
$file = $this->request->getFile('thumbnail');
        dd($file->getName());

First Register service

Code:

$file = new \CodeIgniter\Files\File($path);
$file = $this->request->getFile('thumbnail');
        dd($file->getName());

I tried this，but got en error:

ErrorException

Undefined variable: path