Comment 12 for bug 918019

Here's a hacky workaround for you:

I found a strange entry in ~/.local/share/applications/mimeapps.list

application/octet-stream=gedit.desktop;

This means open almost any kind of binary file in gedit.

Perhaps you have something similar.

Deleting that line has solved the problem for me, until next time.

There is very likely to be a next time; let me explain:

Probably, one day I asked Ubuntu to open a file that I thought was text in gedit, only to get some kind of scrambled nonsense since the file was actually binary.

My particular case aside, opening an unkown file in the wrong application is a very easy mistake to make.

Ubuntu quietly, and insidiously, saved my mistake as a mime-application override (nothing indicates that your one-time choice will become permanent).

Fortunately, this had no apparent affect on my everyday work in Ubuntu (this time, other times it has had an effect.).

How could I have known that firefox gets its half-brained ideas about what applications open what from that list of overrides.