code: public MultipartContent part(String fieldName, String fileName, String contentType, Object content) {
          entries.add(new MultipartPart(fieldName, fileName, contentType, content));
          return this;
      }
docs: Configures a file part with the specified properties. Encoders must be configured on the {@link HttpBuilder} to handle the content type
  of each configured part.
  
  @param fieldName the field name
  @param fileName the file name
  @param contentType the part content type
  @param content the part content (encoders must be configured)
  @return a reference to this {@link MultipartContent} instance