public function onUploadFileStart($in)
    {
        $this->freezeInput();
        FileSystem::tempnam(ini_get('upload_tmp_dir'), 'php', function ($fp) use ($in) {
            if (!$fp) {
                $in->curPart['fp'] = false;
                $in->curPart['error'] = UPLOAD_ERR_NO_TMP_DIR;
            } else {
                $in->curPart['fp'] = $fp;
                $in->curPart['tmp_name'] = $fp->path;
            }
            $this->unfreezeInput();
        });
    }