Once I downloaded a 7z file and it wouldn't open. First I thought that the archive was corrupted so I downloaded it from another mirror and I got the same error.
Then I realised that Ubuntu doesn't come with support for .zip and .7z archives allthough Linux Mint does have support for .zip and .7z by default.
To open .zip and .7z archives in Ubuntu open terminal and type:
sudo apt-get install unrar-freeFor .7z
sudo apt-get install p7zip-full
These small things are annoying for beginners. I'll add them to my installscript for the next version of Ubuntu.