If you use linux, are you only able to download file with .tar extension?

No, you can download anything you want to. The .tar file extension just means that the file is compressed, like a .zip file on Windows.

No limitations; you can download anything you want.

You may or may not be able to run programs written for the Windows environment. But you can download anything.

I have used and still use many different versions of Linux and to be honest, you can download anything on Linux, you can on Windows... (granted as long as there is not a firewall, blocking you) . EXE files for example (not counting "Wine"... I am not going to get into that here) will not be able to be run from Linux, though (just used EXE files for an example) you can download them no problem

-Xmetalfanx