code: String uniqueImagename(String folder, String filename) {
           String buildFileName = filename;//.replace(' ', '_');
  
           while ( (new File(folder , buildFileName)).exists())
               buildFileName = fn.increment();
  
           return buildFileName;//output.getPath();
       }
docs: Calculates a unique filename located in the image upload folder.
  
  WARNING: not threadsafe! Another file with the calculated name might very well get written onto disk first after this name is found
  
  @param filename
  @return Outputs the relative path of the calculated filename.