Testing file uploads

Hi,

Recently this has been added to Laravel:

https://laravel.com/docs/5.4/http-tests#testing-file-uploads

But when I try this I keep getting a phpunit error:

Segmentation fault: 11

My test looks like this:

$user = factory(User::class)->create([
    'email'     => '[email protected]',
    'active'    => 2
]);

factory(Forum::class)->create();

$response = $this->actingAs($user, 'api')->json('POST', '/api/message', [
    'forum_id'      => 1,
    'subject'       => 'test',
    'message'       => 'test,',
    'attachment[0]' => UploadedFile::fake()->image('avatar.jpg')
]);

$response->assertStatus(200);

I'm storing files like this:

if($request->hasFile('attachment')) {
    foreach($request->file('attachment') as $file) {
        $this->addMedia($file)
            ->setName($file->getClientOriginalName())
            ->toMediaLibrary();
    }
}

What could be wrong here? When I remove this line:

'attachment[0]' => UploadedFile::fake()->image('avatar.jpg')

The test returns green.