code: public NetworkRequest build() {
          if (parts.isEmpty()) {
              throw new IllegalStateException("Multipart body must have at least one part.");
          }
          MultipartRequest mrb = new MultipartRequest(type, boundary, parts);
          Buffer bb = new Buffer();
          try {
              mrb.writeOrCountBytes(bb, false);
          } catch (IOException e) {
              e.printStackTrace();
          }
          return new NetworkRequest.Builder()
                  .setContentType(mrb.contentType())
                  .setBody(bb.readByteArray())
                  .build();
      }
docs: Assemble the specified parts into a request body.