BrowserKitTesting test if file was downloaded after form submit

I have a simple form in laravel to convert excel to csv. After i submit my button, a function called convertToCsv will be executed with the given requests. Finally the file will be downloaded and deleted after sending:

return response()
       ->download($destinationFilePath)
       ->deleteFileAfterSend(true);

This works good.

Now i want to test my form if the file was downloaded and is equal to a test csv file.

For this i wrote a PHPUnit test with BrowserKitTesting:

/**
 * @param $init
 * @param $expected
 * @dataProvider convert_to_csv
 */
public function testconvert($init, $expected)
{
    $inputFile = $init['inputFile'];
    if($init['inputFile']) {
        $inputFile = base_path($inputFile);
    }
    $user = $this->auth();
    $this->actingAs($user)
        ->visit($init['url'])
        ->select($init['inputDelimiter'], 'inputDelimiter')
        ->select($init['outputDelimiter'], 'outputDelimiter')
        ->attach($inputFile, 'file');

    $this->press($init['sendButton']);

}

public function convert_to_csv()
{
    return [
        'convert excel to csv' => [
            'init' => [
                'url' => '/converttocsv',
                'inputDelimiter' => ';',
                'outputDelimiter' => ';',
                'headerAlternative' => false,
                'inputFile' => 'path/to/input/file.xlsx',
                'sendButton' => 'Convert'
            ],
            'expected' => [
                'expectedFile' => 'path/to/expected/file.csv'
            ]
        ]
    ];
}

I don't know how i continue after the press function to test with the downloaded file. I get this Error: Expecting a DOMNodeList or DOMNode instance, an array, a string, or null, but got "boolean".