This post describes how to associate a file ending like PAR2-files with the pypar2 application in Ubuntu Unity.
You can also use the mimeopen command to change associated file type to other applications.

$ mimeopen -d unknown-file.par2
Please choose a default application for files of type application/x-par2

	1) Other...

use application #1
use command: /usr/bin/pypar2
Opening "unknown-file.par2" with usr  (application/x-par2)

pypar2 will now open the files automatically when you open it using Nautilus filemanager.

This has been tested on Ubuntu 13.10 and 14.04.