Fix multiple files upload in the same time with temp files handler.

Fix multiple file upload in the same time

Fix dir value expression, which probable came from some old version:
  const dir = options.tempFileDir || process.cwd() + '/tmp/';
insted of
  const dir = __dirname + (options.tempFileDir || '/tmp/');

Use path module to deal with paths to temp directory and temp files.

Change description about necessary trailing slashed in temporary files path.