As an avid user of Sublime Text (absolutely love it!) the fact it doesnt auto-assign a file extension for plain text annoys me! This is mostly because on a Mac all files without a file extension will not preview properly when your navigating the finder.
So to ensure all your sublime text (plain text) files are automatically given a .txt file extension, follow these steps;
Open Sublime Text, and create a new file (COMMAND + N).
Choose preferences, Settings Syntax Specific as shown below;
In the right pane add the following line in between the braces;
Now, when you save your file it should automatically include the .txt extension.