How do I change the default application for unknown file types?

modify ~/.local/share/applications/mimeapps.list

[Default Applications] 
- text/plain=bless.desktop
- application/octet-stream=bless.desktop
+ application/octet-stream=gedit.desktop

This reverts the text/plain app to using the system default in /usr/share/applications/defaults.list, and changes the default application/octet-stream. It seems that Firefox labels this mime-type as "Unknown".

right click on the unknown file, properties, open with. If your desired application is in the list, click on it, if not then click on the Show Other Applications and select the one you want. Cheers!