code: public static void putMetaDatas(Map<String, Object> metadatas, String contentType, String fileName){
          metadatas.put(MULTIPART_FLAG, true);
          if(isNotBlank(contentType)) {
              metadatas.put(CONTENT_TYPE, contentType);
          }
          if(isNotBlank(fileName)) {
              metadatas.put(FILENAME, fileName);
          }
      }
docs: Puts multipart metadata informations into the given metadata map
  @param metadatas map to put the metadata into
  @param contentType multipart content-type. can be null or empty
  @param fileName multipart file name. can be null or empty