code: def DownloadFile(file_obj, target_path, buffer_size=BUFFER_SIZE):
    
    logging.info(u"Downloading: %s to: %s", file_obj.urn, target_path)
  
    target_file = open(target_path, "wb")
    file_obj.Seek(0)
    count = 0
  
    data_buffer = file_obj.Read(buffer_size)
    while data_buffer:
      target_file.write(data_buffer)
      data_buffer = file_obj.Read(buffer_size)
      count += 1
      if not count % 3:
        logging.debug(u"Downloading: %s: %s done", file_obj.urn,
                      utils.FormatNumberAsString(count * buffer_size))
    target_file.close()
docs: Download an aff4 file to the local filesystem overwriting it if it exists.
  
    Args:
      file_obj: An aff4 object that supports the file interface (Read, Seek)
      target_path: Full path of file to write to.
      buffer_size: Read in chunks this size.