docs: public void addFilePart(final String fieldName, final InputStream stream, final String fileName, final String contentType)
          throws IOException
      {
          writer.append("--").append(boundary).append(LINE_FEED);
  
          writer.append("Content-Disposition: form-data; name=\"").append(fieldName).append("\"");
          if (fileName != null) {
              writer.append("; filename=\"").append(fileName).append("\"");
          }
          writer.append(LINE_FEED);
  
          writer.append("Content-Type: ").append(contentType).append(LINE_FEED);
          writer.append("Content-Transfer-Encoding: binary").append(LINE_FEED);
          writer.append(LINE_FEED);
          writer.flush();
  
          OsUtils.copyStreams(stream, outputStream);
  
          outputStream.flush();
  
          writer.append(LINE_FEED);
          writer.flush();
      }