I use java to send a fax via the RingCentral Fax API. However I got HTTP 415 error code. I really have no idea how to make it work.

public void sendFax(String body, String...toList) {

    CloseableHttpClient httpClient = HttpClients.createDefault();

    HttpPost httpPost = new HttpPost(baseUrl + URI_FAX);

    httpPost.addHeader("Content-Type", "multipart/mixed; boundary=Boundary_1_6506429161_101");

    httpPost.addHeader("Authorization", "Bearer " + rcoAuthResp.getAccess_token());



    try {

        List < RCPhone > list = new ArrayList < > ();

        for (String to: toList) {

            list.add(RCPhone.builder().phoneNumber(to).build());

        }

        String requestBody = mapper.writeValueAsString(RCFaxPayload.builder().to(list).faxResolution("High").build());



        FormBodyPart bodyPart1 = FormBodyPartBuilder.create()

            .setName("json")

            .setBody(new StringBody(requestBody, ContentType.APPLICATION_JSON))

            .build();

        bodyPart1.getHeader().removeFields("Content-Disposition");

        bodyPart1.getHeader().removeFields("Content-Transfer-Encoding");

        bodyPart1.getHeader().removeFields("Content-type");

        bodyPart1.addField("Content-type", "application/json");



        FormBodyPart bodyPart2 = FormBodyPartBuilder.create()

            .setName("body")

            .setBody(new StringBody(body, ContentType.TEXT_PLAIN))

            .build();

        bodyPart2.getHeader().removeFields("Content-Disposition");

        bodyPart2.getHeader().removeFields("Content-Transfer-Encoding");

        bodyPart2.getHeader().removeFields("Content-type");

        bodyPart2.addField("Content-type", "text/plain");



        HttpEntity entity = MultipartEntityBuilder

            .create()

            .setBoundary("Boundary_1_6506429161_101")

            .addPart(bodyPart1)

            .addPart(bodyPart2)

            .build();



        httpPost.setEntity(entity);

        httpPost.getEntity().writeTo(System.out);



        String response = processHttpResp(httpClient, httpPost);

        System.out.println(response);

    } catch (Exception e) {

        log.error(e.getMessage(), e);

    }

}