synapse PHP API sandbox mode showing error "None is not of type 'string'"

I am working with synapse and having issue while uploading user doc attachment, actually same thing is working on our live server but not working with test server in sandbox mode If any one can help :)

Reference URL: https://docs.synapsepay.com/v3/docs/attach-document

API URL: https://sandbox.synapsepay.com/api/3/user/doc/attachments/add

Response: Array ( [error] => Array ( [en] => None is not of type 'string' ) [error_code] => 200 [http_code] => 400 [success] => )

Code as Below:

$url = "https://sandbox.synapsepay.com/api/v3/user/doc/attachments/add";

 // KYC Documentation

$payload = array(
    "login" => array(
        //Oauth_key of the user to add KYC doc
        "oauth_key" => "hTUCH4kO89qGZDpyEdoq55ODYugwwRsd57ti8ohZ"
    ),
    "user" => array(
        //doc data
        "doc" => array(
            'attachment' => 'data:text/csv;base64,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'
        ),
        "fingerprint" => "suasusau21324redakufejfjsf",
    )
);

$options = array(
    'http' => array(
        'method' => 'POST',
        'content' => json_encode( $payload ),
        'header' => "Content-Type: application/json\r\n" .
            "Accept: application/json\r\n"
    )
);

$data_string = json_encode($payload);

curlIT($url, $data_string);

function curlIT($url, $fields){
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
    curl_setopt($ch, CURLOPT_POSTFIELDS, $fields);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_HTTPHEADER, array(
        'Content-Type: application/json',
        'Accept: application/json')
    );

    $result = curl_exec($ch);

    print_r($result); 
}

exit;

?>

Solution

Synapse api changed you need to send auth key and client id in headers Take a look of new document https://docs.synapsepay.com/docs/adding-documents