code: private byte[] startNextFile() {
  		byte[] ret;
  		if (currentSource == softDeviceBytes && bootloaderBytes != null && (type & DfuBaseService.TYPE_BOOTLOADER) > 0) {
  			ret = currentSource = bootloaderBytes;
  		} else if (currentSource != applicationBytes && applicationBytes != null && (type & DfuBaseService.TYPE_APPLICATION) > 0) {
  			ret = currentSource = applicationBytes;
  		} else {
  			ret = currentSource = null;
  		}
  		bytesReadFromCurrentSource = 0;
  		return ret;
  	}
docs: Sets the currentSource to the new file or to <code>null</code> if the last file has been
  transmitted.
  
  @return The new source, the same as {@link #currentSource}.