code: public MultipartContent part(String fieldName, String contentType, String value) {
          return part(fieldName, null, contentType, value);
      }
docs: Configures a field part with the given field name and value (of the specified content type).
  
  @param fieldName the field name
  @param contentType the value content type
  @param value the value
  @return a reference to this {@link MultipartContent} instance