code: protected OkRequest<T> writePartHeader(final String name,
                                             final String filename, final String contentType) throws IOException {
          final StringBuilder partBuffer = new StringBuilder();
          partBuffer.append("form-data; name=\"").append(name);
          if (filename != null) {
              partBuffer.append("\"; filename=\"").append(filename);
          }
          partBuffer.append('"');
          partHeader("Content-Disposition", partBuffer.toString());
          if (contentType != null) {
              partHeader(HEADER_CONTENT_TYPE, contentType);
          }
          return send(CRLF);
      }
docs: Write part header
  
  @param name
  @param filename
  @param contentType
  @return this request
  @throws java.io.IOException