UPDATE: The real way
In Coda, open Preferences, go to Editor, at the bottom, add a Custom Syntax Mode.
As per this guide:
- Right click on
Coda.app
in /Applications
and select Show Package Contents
- Navigate to
Contents/Resources/Modes/HTML.mode/Contents/Resources
- Open ModeSettings.xml
- Add
<extension>ss</extension>
to the list of extensions.
Or, make your mode (eg, SilverStripe.mode) and put it in ~/Library/Application Support/Coda/Modes
Or, copy HTML.mode to ~/Library/Application Support/Coda/Modes
and then modify it, to preserve the original.
(For system-wide changes without modifying the application bundle, create the directory /Library/Application Support/Coda/Modes
and put .mode directories there.)