<?php                                   

$filename = "/Path to the file/vat_invoice.pdf";

$client = new \GuzzleHttp\Client();
        
$response = $client->post(
    'https://api.dotcon.io/api/v1/allegro/upload-vat-invoice',
    [
        'headers' => [
            'Authorization' => 'Bearer your authentication token',
            'Accept' => 'application/json',
        ],
        'multipart' => [
            [
                'name'     => 'order_id',
                'contents' => '31e88c80-a380-11ef-aaa8-ad165b9e76d9',
            ],
            [
                'name'     => 'invoice_number',
                'contents' => 'AB-AB-ABC-1234-1234567',
            ],
            [
                'name'     => 'vat_invoice',
                'contents' => fopen($filename , 'r'),
		'filename' => basename($filename),
            ],
        ]
    ]
);

$body = $response->getBody();
print_r(json_decode((string) $body));

-----------------------------------------------------------------

Successful response:

{"message":"File saved successfully."}