code: public MultipartBuilder type(MediaType type) {
          if (type == null) {
              throw new NullPointerException("type == null");
          }
          if (!type.type().equals("multipart")) {
              throw new IllegalArgumentException("multipart != " + type);
          }
          this.type = type;
          return this;
      }
docs: Set the MIME type. Expected values for {@code type} are {@link #MIXED} (the
  default), {@link #ALTERNATIVE}, {@link #DIGEST}, {@link #PARALLEL} and
  {@link #FORM}.