File Upload schlug fehl / File upload failed

function uploadDetailImages($product, $product_id, $post){

$blaa = $_FILES['product_image_detailimage'];

foreach ($blaa as $key => $value) {
$app->enqueueMessage('key: '.$key.' value: '.$value, 'message');
}
/*
Ausgabe von enqueueMessage
key: name value:
key: type value:
key: tmp_name value:
key: error value: 4
key: sizev alue: 0
*/
$jshopConfig = JSFactory::getConfig();
$dispatcher = JDispatcher::getInstance();

$upload4 = new UploadFile($_FILES['product_image_detailimage']);
$upload4->setAllowFile(array('jpeg','jpg','gif','png'));
$upload4->setDir($jshopConfig->image_product_path);
$upload4->setFileNameMd5(0);
$upload4->setFilterName(1);
if ($upload4->upload()){
$name_image = $upload4->getName();
@chmod($jshopConfig->image_product_path."/".$name_image, 0777);
}else{
if ($upload4->getError() != 4){
JError::raiseWarning("", "ist kein Fehler, Bild wurde hochgeladen...");
saveToLog("error.log", "SaveProduct - Error upload image. code: ".$upload4->getError());
}
}
unset($upload4);
}

$blaa = $_FILES['product_image_detailimage'];

foreach ($blaa as $key => $value) {
$app->enqueueMessage('key: '.$key.' value: '.$value, 'message');
}
/*
Ausgabe von enqueueMessage
key: name value:
key: type value:
key: tmp_name value:
key: error value: 4
key: sizev alue: 0
*/