Использую вот этот image uploader: 
https://github.com/perminder-klair/yii2-dropzone
Настройки расширения : 
echo \kato\DropZone::widget([
        'options' => [
            'maxFilesize' => '1',
            'uploadMultiple' => true,
            'url' => '/admin/works/upload',
            'addRemoveLinks' => true,
        ],
        'clientEvents' => [
            'complete' => "function(data){console.log(data)}"
        ],
    ]);
Метод : 
public function actionUpload()
    {
        $file = UploadedFile::getInstanceByName('file');
        $file->saveAs($_SERVER['DOCUMENT_ROOT'] . '/frontend/web/uploads/works/' . $file->baseName . '.' . $file->extension);
        return true;
    }
Ошибка в консоли : 
Failed to load resource: the server responded with a status of 500 (Internal Server Error)
<pre>PHP Fatal Error 'yii\base\ErrorException' with message 'Call to a member function saveAs() on null' ↵↵in /home/alexander/My_projects/spa.local/backend/controllers/WorksController.php:67↵↵Stack trace:↵#0 [internal function]: yii\base\ErrorHandler->handleFatalError()↵#1 {main}</pre>"
ошибка понятна $file - пустая , но по умалчанию как сказанно "paramName = default file". 
www.dropzonejs.com/#configuration-options