I have a lot of files on my system without extensions (i.e., files like
list_of_stuff, etc). I would like these types of files to default to
markdown-mode for their major mode. How to do this? In case it matters, I'm running spacemacs.
From the Emacs "Major Modes" info topic (C-h i d m emacs m major), try adding this to your spacemacs user-config:
(setq-default major-mode 'markdown-mode)
The "Choosing Modes" info will be a good resource if you need more nuance.