Integrated - Attach multiple files

I'm working on a form, where you have the option to upload multiple files. This is how it look like in the view file:

    <form action="{{ route('admin.media.store') }}" enctype="multipart/form-data"  accept-charset="UTF-8" method="post">
        {{ csrf_field() }}
        <input type="file" name="files[]" id="upload-files" multiple="multiple" />
        <button type="submit" class="btn btn-primary" id="btn-media-upload">Upload files</button>
    </form>

Please note that the file input field's name is files[].

It' working well in the browser, but I don't know how to write the test for this form with Integrated.

If I try this way:

        $this->visit('/admin/media')
             ->attach($filePath, 'files[]')
             ->press('Upload files');

I get the following error while running the test:

PHPUnit 4.8.24 by Sebastian Bergmann and contributors.

E

Time: 5.44 seconds, Memory: 38.50Mb

There was 1 error:

1) AdminManageMediaTest::an_admin_should_be_able_to_upload_an_image_file_to_the_storage
InvalidArgumentException: Unreachable field ""

/Users/jpapp/Code/Personal/vendor/symfony/dom-crawler/FormFieldRegistry.php:89
/Users/jpapp/Code/Personal/vendor/symfony/dom-crawler/FormFieldRegistry.php:126
/Users/jpapp/Code/Personal/vendor/symfony/dom-crawler/Form.php:77
/Users/jpapp/Code/Personal/vendor/laravel/framework/src/Illuminate/Foundation/Testing/Concerns/InteractsWithPages.php:576
/Users/jpapp/Code/Personal/vendor/laravel/framework/src/Illuminate/Foundation/Testing/Concerns/InteractsWithPages.php:556
/Users/jpapp/Code/Personal/vendor/laravel/framework/src/Illuminate/Foundation/Testing/Concerns/InteractsWithPages.php:543
/Users/jpapp/Code/Personal/tests/features/AdminManageMediaTest.php:46

If I try to use the id of the file input:

        $this->visit('/admin/media')
             ->attach($filePath, '#upload-files')
             ->press('Upload files');

I get the following error:

PHPUnit 4.8.24 by Sebastian Bergmann and contributors.

E

Time: 1.61 seconds, Memory: 38.50Mb

There was 1 error:

1) AdminManageMediaTest::an_admin_should_be_able_to_upload_an_image_file_to_the_storage
InvalidArgumentException: Unreachable field "upload-files"

/Users/jpapp/Code/Personal/vendor/symfony/dom-crawler/FormFieldRegistry.php:89
/Users/jpapp/Code/Personal/vendor/symfony/dom-crawler/FormFieldRegistry.php:126
/Users/jpapp/Code/Personal/vendor/symfony/dom-crawler/Form.php:77
/Users/jpapp/Code/Personal/vendor/laravel/framework/src/Illuminate/Foundation/Testing/Concerns/InteractsWithPages.php:576
/Users/jpapp/Code/Personal/vendor/laravel/framework/src/Illuminate/Foundation/Testing/Concerns/InteractsWithPages.php:556
/Users/jpapp/Code/Personal/vendor/laravel/framework/src/Illuminate/Foundation/Testing/Concerns/InteractsWithPages.php:543
/Users/jpapp/Code/Personal/tests/features/AdminManageMediaTest.php:46

Any idea how to test the multiple file upload fields?