@Test
  public void postMultipartWithContentType() throws Exception {
    final AtomicReference<String> body = new AtomicReference<String>();
    handler = new RequestHandler() {

      @Override
      public void handle(Request request, HttpServletResponse response) {
        response.setStatus(HTTP_OK);
        body.set(new String(read()));
      }
    };
    HttpRequest request = post(url);
    request.part("body", null, "application/json", "contents");
    assertTrue(request.ok());
    assertTrue(body.toString().contains("Content-Type: application/json"));
    assertTrue(body.toString().contains("contents\r\n"));
  }
Verify multipart with content type part header

@throws Exception
lib/src/test/java/com/github/kevinsawicki/http/HttpRequestTest.java