How to upload image in codeigniter 4

Pls. help;
//view
<?=form_open_multipart( '', 'class="user_form" id="user_form"' ) ?>
<div class="form-group">
            <label for="avatar"> Avatar </label>
            <input class="form-control" type="file" name="avatar" id="avatar" />
        </div>

<button type="button" class="btn  btn-primary" onclick="add_user( '<?=$id?>')">Save changes</button>
<?=form_close();?>

<script>
function add_user( id = '') {
        var user_data    = $( '.user_form' ).find( 'input, select, button' );
        var form_data    = user_data.serialize();
        var form_data    = form_data + '&<?php echo csrf_token(); ?>:<?php echo csrf_hash(); ?>&id=' + id;
       
        $.ajax({
            type        : 'POST',
            url        : '<?=site_url( 'public/home/add_user' );?>',
            data        : form_data,
            async      : false,
            dataType    : 'json',
            success    : function( msg ) { }

})
}
</script>

//controller

function add_user() {
     $avatar = $this->request->getFile( 'avatar' );
      $avatar->move( WRITEPATH. 'uploads' );
}

result

i did not get any file upload.and it show Array()

pls help me