VK API: ошибка "One of the parameters specified was missing or invalid: photos_list is invalid"

$PATH="D:\\2.jpg";
/*get server for upload to wall step1*/
$Request_step1 ="https://api.vkontakte.ru/method/photos.getWallUploadServer?group_id=$group_id&access_token=$token";
$Responce_step1 = json_decode(file_get_contents($Request_step1));
  var_dump(json_decode($Request_step1));  
  var_dump ($Responce_step1);
 
/*determine url_server*/
$imgserv=$Responce_step1->response->upload_url;
echo $imgserv;
 
/*making post param for step2*/
$postparam=array("file1"=>"@".$PATH);
  var_dump($postparam) ;
 
/*making post step2*/
 
$ch=curl_init();
curl_setopt($ch, CURLOPT_URL,$imgserv);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS,$postparam);
$Request_step2 = curl_exec($ch);
curl_close($ch);
 
/*recieving data from vk of uploaded file*/
$Responce_step2=json_decode($Request_step2);
var_dump ($Responce_step2);
var_dump (json_decode($Responce_step2->photo));
 
/*save img at vk*/
$imgdata=file_get_contents("https://api.vkontakte.ru/method/photos.saveWallPhoto?user_id=$id&photo=$Responce_step2->photo&server=$Responce_step2->server&hash=$Responce_step2->hash&access_token=$token");
$step3=json_decode($imgdata);
var_dump (json_decode($imgdata));
 
/*determine media name in vk*/
$media=$step3->response[0]->id;
echo $media;
 
/*post on wall*/
$sRequest ="https://api.vkontakte.ru/method/wall.post?owner_id=-$group_id&attachments=$media&access_token=$token";
$fin=file_get_contents($sRequest);