PHP. Send file to endpoint as binary data [требует правки]

In the example below, the file is sent as a string. Is it possible to send a file as a binary data via Guzzle? I mean, send the file as is, without reading.

$client->request('POST', '/post', [
    'multipart' => [
        [
            'name'     => 'foo',
            'contents' => 'data',
            'headers'  => ['X-Baz' => 'bar']
        ],
        [
            'name'     => 'baz',
            'contents' => fopen('/path/to/file', 'r')
        ],
        [
            'name'     => 'qux',
            'contents' => fopen('/path/to/file', 'r'),
            'filename' => 'custom_filename.txt'
        ],
    ]
]);