Default file associations are missing on Jessie

Originally created by @intrigeri on #10798 (Redmine)

What I see instead is a “Launch Application” dialog box. Manually choosing /usr/local/bin/tor-browser
opens the link in Tor Browser.

In 1.8.1 I see the “Launch Application” dialog box, and Tor Browser is the default choice, which is much better indeed.

Feature Branch: bugfix/10798-fix-file-associations

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