think\File::buildSaveName PHP Method

    protected function buildSaveName($savename)
    {
        if (true === $savename) {
            // 自动生成文件名
            if ($this->rule instanceof \Closure) {
                $savename = call_user_func_array($this->rule, [$this]);
            } else {
                switch ($this->rule) {
                    case 'date':
                        $savename = date('Ymd') . DS . md5(microtime(true));
                        break;
                    default:
                        if (in_array($this->rule, hash_algos())) {
                            $hash = $this->hash($this->rule);
                            $savename = substr($hash, 0, 2) . DS . substr($hash, 2);
                        } elseif (is_callable($this->rule)) {
                            $savename = call_user_func($this->rule);
                        } else {
                            $savename = date('Ymd') . DS . md5(microtime(true));
                        }
                }
            }
        } elseif ('' === $savename) {
            $savename = $this->getInfo('name');
        }
        if (!strpos($savename, '.')) {
            $savename .= '.' . pathinfo($this->getInfo('name'), PATHINFO_EXTENSION);
        }
        return $savename;
    }