code: public OkRequest<T> part(final String name, final String filename,
                               final String contentType, final File part) throws IOException {
          final InputStream stream;
          stream = new BufferedInputStream(new FileInputStream(part));
  //            incrementTotalSize(part.length());
  
          return part(name, filename, contentType, stream);
      }
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