code: public OkRequest<T> part(final String name, final String filename,
                               final String contentType, final String part) throws IOException {
  
          startPart();
          writePartHeader(name, filename, contentType);
          mOutput.write(part);
  
          return this;
      }
docs: Write part of a multipart request to the request body
  
  @param name
  @param filename
  @param contentType value of the Content-Type part header
  @param part
  @return this request