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

Ever since installing Bless Hex Editor, all unknown files and text files automatically open with Bless instead of gedit. How do I change it back? I don't want to have to manually associate files with gedit, I just want gedit to be the default if there's no other application set up to open the file.

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

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

Solution 1:

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".

Solution 2:

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!