i had installed multisites in the wrong site so i was uninstalling it but now it says Error when restoring the Joomla core files.
The files that are not correctly restored are:
- libraries/joomla/filesystem/folder.php
can i recover this.?
please help

YES you could but this is not critical.
Which version of Joomla are you using.
Starting with Joomla 1.5.10, the joomla team has included our patch in their distribution to fix a bug in Joomla.
So restore this file for a Joomla 1.5.10 or higher is not critical.

Even if you have a Joomla before 1.5.10, this means that the bug fix remain in Joomla.
As this is a bug fix that we have included in JMS, you can ignore the error message.

In most frequent case, when you have a problem to restore an original file, this is not critical as JMS is designed to do nothing when some variable are not enabled.
This is like that that JMS works as a master website (it does nothing).