Stop folders opening with different application than the file manager

Answer edited out from the question

Solution was obvious, in Nautilus I select a folder, right click, Open with other application, and pick "Files" which is apparently the name for Nautilus. Now that set the default back... until next time! I guess this is a cautionary tale.

In your home folder go to .local/share/applications/mimeapps.list It is a hidden file. You can unhide it with the following keyboard shortcut: Ctrl + H .

You then need to modify the line inode/directory=movie-player.desktop; or something like that, to inode/directory=nautilus-folder-handler.desktop; and save the file.

On Ubuntu 14.04, this one works for me:

xdg-mime default nautilus.desktop inode/directory

Original answer comes from here and here

The command above adds a line inode/directory=nautilus.desktop to your .local/share/applications/mimeapps.list

Works instantly without logging out/restart.