Submit post params with Files using CURL Call

I am trying to submit some post variables along with some files on 3rd party URL via CURL call.

For that i have used ajax from where i am passing whole form data on controller and from controller, i am calling CURL with form data.

but unfortunately those data is not passed to 3rd party URL.

here is controllers code
`

     $clientId = $this->request->data['clientSsoCert'];

     //file details which i wants to upload
     $xmlTmpPath = $this->request->data['clientSsoXml']['tmp_name'];
     $xmlType = $this->request->data['clientSsoXml']['type'];
     $xmlName = $this->request->data['clientSsoXml']['name'];
     $xmlNameCmps = explode(".", $xmlName);
     $xmlExtension = strtolower(end($xmlNameCmps));

CURL calls logic

$strRequestURL = 'https://abcd.com';
    try {
        $conn = curl_init($strRequestURL);

        curl_setopt($conn, CURLOPT_URL, $strRequestURL);
        curl_setopt($conn, CURLOPT_RETURNTRANSFER, true);
        curl_setopt($conn, CURLOPT_POST, true);
        curl_setopt($conn, CURLOPT_HTTPHEADER, array('Content-Type:multipart/form-data'));
        curl_setopt($conn, CURLOPT_POSTFIELDS, ['custId' => $clientId , 'xmlfile' => $xmlTmpPath]);
        curl_setopt($conn, CURLOPT_SSL_VERIFYPEER, false);
        curl_setopt($conn, CURLOPT_SSL_VERIFYHOST, false);

        $output = curl_exec($conn);

        if (curl_errno($conn)) {
            return ['status' => false, 'error' => curl_error($conn)];
        } else {
            $response = json_decode($output);
            if ($response->status == 'error') {
                return ['status' => false, 'error' => $response->errorMessage];
            } else {
                return ['status' => true, 'xmlFileName' => ($response->xmlfileName) ?? "", 'certFileName' => ($response->certfileName) ?? ""];
            }
        }
        curl_close($conn);
    } catch (Exception $e) {
        return ['status' => false, 'error' => $e->getMessage()];
    }

Above codes returns error which i shows on SS.

I am not sure how to upload image using CURL so any suggestions for that ?
i have used

        curl_setopt($conn, CURLOPT_HTTPHEADER, array('Content-Type:multipart/form-data'));

and also the same on html form.
while calling the same API via postman without passing any params the same error i got, from there i found my params are not reaching there, thats why that error is showing.