public function rules()
    {
        return [
            'img.*.name'   => [
                'required_with:img.*.file',
                'string',
                'max:50',
            ],
            'img.*.file'   => [
                'file',
                'image',
                'mimes:jpeg,jpg,png,gif',
                'max_mb:10',
            ],
        ];
    }

    public function withValidator($validator) {
        $validator->after(function ($validator) {
            $file_count = 0;

            foreach ($this->file('img') as $key => $img) {
                if (isset($img['file']) && is_uploaded_file($img['file'])) {
                    $file_count ++;
                }
            }

            if ($file_count === 0) {
                $validator->errors()->add('img', '画像は必須です。');
            }
        });
    }