Update: this is now much easier to do:
- Open the .ssi file
- In the status bar (lower-right), click the dropdown that says "Text"
- Choose a different option (HTML in this case)
- Open the dropdown again and choose the "Set as Default" option at the top
Original answer:
You don't have to clone Brackets from git as suggested in the other SO answer - it was only needed because the file extension in that case was already assigned to a different language (which isn't that common).
For unassigned file extensions like .ssi, you can create a very simple Brackets extension - just a main.js file containing this code:
define(function (require, exports, module) {
var LanguageManager = brackets.getModule("language/LanguageManager");
var language = LanguageManager.getLanguage("html");
language.addFileExtension("ssi");
});
This is easier to set up than pulling from git, and a little more foolproof. (In the future, this should be more even easily configurable though).
For more info, see this answer: how to add file extension in adobe-brackets editor ?