Comment 26 for bug 918019

I worked around this bug by adding a .desktop file for xdg-open and then setting it to be the default handler for unknown binary (application/octet-stream) files. It appears as "Let Ubuntu choose" in Firefox for opening files. I haven't had any problems since.

1. Put the below lines into a new file /usr/share/applications/xdg-open.desktop (using sudo)
[Desktop Entry]
Name=Let Ubuntu choose
Exec=/usr/bin/xdg-open
Terminal=false
Type=Application
NoDisplay=true

2. Edit .local/share/applications/mimeapps.list and replace this line:
application/octet-stream=gedit.desktop;
...with this:
application/octet-stream=xdg-open.desktop;