Uploading files to VK. Via VK API. What am I doing wrong?

Question

PHP

Good afternoon!
There is a download code:

$server = $this->request('photos.getWallUploadServer', [
    'group_id'  =>  $groupId
]);

$uploadDir = $server->upload_url;

$ch = curl_init($uploadDir);
curl_setopt($ch, CURLOPT_HTTPHEADER, array("Content-type: multipart/form-data"));
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
curl_setopt($ch, CURLOPT_USERAGENT, "Mozilla/4.0 (compatible;)");
curl_setopt($ch, CURLOPT_POSTFIELDS,  ['photo' => '@'.realpath('C:/www/OpenServer/domains/srv/web/images/wall/1/12.jpg')]);
$otvet = curl_exec($ch);
curl_close($ch);
var_dump($otvet);

For some reason, the file is not loaded, the response comes:
"{"server":627922,"photo":"[]","hash":"578a004d576a5a13d20f24b3669de74a"}"
I get the Url for downloading by the method:
https://vk.com /dev/photos.getWallUploadServer I have it correct
And I upload the file using the method: Uploading photos to the user's wall
: passed user_id or group_id parameters.
2. The application generates a POST request to the received address.
The request must include the photo field containing the image file (JPG, PNG,