If i upload via ftp a .tar file using my username then go to telnet root to untar it who will the files owner be: the user or the root?

Unless I'm completely mistaken the files will belong to the user that uploaded, downloaded or created them....
In any case you can easily change ownership using the chown command eg.chown filename newusername

It will be root, if i'm not wrong.

Yes, you're right (I since tested that).Of course he can change it back. If it's a problem, why not untar it as the non-root user?