error api for uploading image of course

<?php

 $token = 'xxxxxxxxxxxx';

 $domainname = 'xxxxxxxxxxxxxxxxxxx';

 $restformat = 'json';

require_once('curl.php'); //Download this file

$curl = new curl;

$path = 'moodle.png';

$type = pathinfo($path, PATHINFO_EXTENSION);

$data = file_get_contents($path);

$base64 = 'data:image/' . $type . ';base64,' . base64_encode($data);

$params = array(

    'component' => 'course',

    'filearea' => 'overviewfiles',

    'itemid' => 0,

    'filepath' => '/',

    'filename' => 'moodle1.png',

    'filecontent' => $base64,

    'contextlevel' => 'course',

    'instanceid' => 2,

);

/// UPLOAD IMAGE - Moodle 3.10+ and later

$functionname = 'core_files_upload';

$serverurl = $domainname .'/webservice/rest/server.php' . '?wstoken=' . $token .'&wsfunction=' . $functionname;

$restformat = ($restformat == 'json') ?

'&moodlewsrestformat=' . $restformat : '';

//

printf("\n");

printf("\n");

$resp = $curl->post($serverurl . $restformat, $params);

$resps = json_decode($resp);

print_r($resps);

printf("\n");

========================

the response is 

stdClass Object ( [exception] => coding_exception [errorcode] => codingerror [message] => Coding error detected, it must be fixed by a programmer: File can be uploaded to user draft area only )

==================

why???