Send Fax doesn't work - I get the error: "Unsupported Media Type"

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);
    }
}