you have got a rar file (or other file formats not naively supported by Ubuntu’s archive manager- such as arj, 7zip, etc), you need to install extra packages so that Archive Manager can handle these file formats. For example, if you would like to unrar a file in Ubuntu, you need to install the package unrar. Open a terminal (command window) and simply issue this command:
sudo apt-get install unrar-free
for a non-free option, you can use:
sudo apt-get install unrar
You might be asked to give your password and then the package is downloaded and installed. After the package is installed, you can use the usual Archive Manager to unrar files.
