ZF2 Validator File by \Zend\File\Transfer\Adapter\Http - zend-framework2

Question

I have HTML Code 2 input file in view of a action.
<form action="#" id="appForm" name="appForm"
enctype="multipart/form-data" method="post">
<input type="file" name="picture" id="picture">
<input type="file" name="attach_file" id="attach_file">
</form>
After that, i submit this form to a action of controller. Code in action to get Post
if($this->getRequest()->isPost()) {
$item = array_merge_recursive($this->params()->fromPost(), $this->params()->fromFiles());
$item['id'] = $this->params()->fromRoute('id');
$item['action'] = 'edit';
$validator = new ArticleValidator($item);
if($validator->isError()) {
$error = array_merge($error, $validator->getError());
$item = $validator->getData();
}else{
echo ' save data';
die();
}
}
And code in ArticleValidator
namespace Admin\Form;
class ArticleValidator {
protected $_arrError = null;
protected $_arrData;
public function __construct($arrParam = array(), $options = null) {
// avatar
if (!empty($arrParam['picture']['name'])) {
$size = new \Zend\Validator\File\Size(array('min' => 1, 'max' => 2097152));
$ext = new \Zend\Validator\File\Extension('gif,jpg,jpeg,png');
$adapterPicture = new \Zend\File\Transfer\Adapter\Http();
$adapterPicture->setValidators(array($size, $ext), $arrParam['picture']['name']);
if (!$adapterPicture->isValid()) {
$message = $adapterPicture->getMessages();
$this->_arrError['picture'] = current($message);
$arrParam['picture'] = '';
}
}
// attach_file
if (!empty($arrParam['attach_file']['name'])) {
$size = new \Zend\Validator\File\Size(array('min' => 1, 'max' => 5242880));
$ext = new \Zend\Validator\File\Extension('rar,zip,gz,docx,pdf,xlsx');
$adapterFile = new \Zend\File\Transfer\Adapter\Http();
$adapterFile->setValidators(array($size, $ext), $arrParam['attach_file']['name']);
if (!$adapterFile->isValid()) {
$message = $adapterFile->getMessages();
$this->_arrError['attach_file'] = current($message);
$arrParam['attach_file'] = '';
}
}
}
}
When i post a valid image into 'picture' field. $this->_arrError will have "File '' was not uploaded". I check and know that, $adapterPicture also check 'attach_file' field. Because i do not post a file into 'attach_file', so it say not uploaded ( for attach_file).
So, how i can check only for 'picture' field and then check only for 'attach_file'.
PS: I'm not use Zend\Form. Just a HTML code with 2 input type=file. And then, check file use \Zend\File\Transfer\Adapter\Http