In the past when I've wanted to change the default application for a file type at the system level I've done this:
# open pdfs with evince not adobe reader by default 'cos it's way faster
sed -i 's:^application/pdf=.*:application/pdf=evince.desktop:' /usr/share/applications/defaults.list
Which means that the default application to open a PDF for all users is now Evince rather than Adobe Reader. It works but seems a bit hack-ish.
I'm wondering is there a better way of achieving this. I haven't been able to find any tools which are designed to manipulate the /usr/share/applications/defaults.list Anyone know of a neater way?
I've found xdg-mime which can be used like this
$ xdg-mime default evince.desktop application/pdf
But that works on a per-user basis not a system wide basis.