[ticket/12560] Add methods to set upload and temp paths in plupload

The paths will be reset to the default paths after an upload is complete.

Ticket: https://tracker.phpbb.com/browse/PHPBB3-12560
What do you mean? Why I reset to the default directories? I thought that once we set the directories in the service other files and even the phpBB attachment upload will also upload to the new directories if we don't reset to the defaults after uploading.