Comment 10 for bug 553256

The icon is system-file-manager-panel.

You should be able to do something like this as a workaround:
sudo ln -s /usr/share/icons/ubuntu-mono-dark/status/22/weather-fog.svg /usr/share/icons/ubuntu-mono-dark/status/22/system-file-manager-panel.svg

But that didn't work for me so I installed elementary-mono-dark from here: http://danrabbit.deviantart.com/art/elementary-Icons-65437279
(And I replaced the existing system-file-manager-panel.svg with weather-fog.svg).