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.