docs: public RequestBody serialize(Object obj, String contentType) throws ApiException {
          if (obj instanceof byte[]) {
              // Binary (byte array) body parameter support.
              return RequestBody.create(MediaType.parse(contentType), (byte[]) obj);
          } else if (obj instanceof File) {
              // File body parameter support.
              return RequestBody.create(MediaType.parse(contentType), (File) obj);
          } else if (isJsonMime(contentType)) {
              String content;
              if (obj != null) {
                  content = json.serialize(obj);
              } else {
                  content = null;
              }
              return RequestBody.create(MediaType.parse(contentType), content);
          } else {
              throw new ApiException("Content type \"" + contentType + "\" is not supported");
          }
      }