Set default language for new files
atom-editor
Solution
Had this problem as well, there is a plugin called default-language that will do this for you.
Search atom for default-language, install and open its settings. Type the name of the language you want Atom to default to, e.g. Shell Script (if in doubt, copy from the language selection menu) in the Default Language field. Next time you open a script with no extension (or shebang) it'll default to the language you set.
Problem
Newly created files in Atom are always "Plain Text". How can I change this so that new files will automatically be in another language, for example "Shell Script (Bash)"? I want to do this because auto indentation does not work with Plain Text files.