[WIP] Fix issue with file types in settings.json (#3915)

* Initial plan

* Add test fixture for multi-language settings investigation

Co-authored-by: ntotten <282782+ntotten@users.noreply.github.com>

* Document VS Code limitation with combined language settings for editor.defaultFormatter

Co-authored-by: ntotten <282782+ntotten@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: ntotten <282782+ntotten@users.noreply.github.com>

Author: Copilot

README.md

@@ -79,6 +79,27 @@ To ensure that this extension is used over other extensions you may have install
 }
 ```
 
+**Note:** VS Code does not support combined language syntax for `editor.defaultFormatter`. You must set the formatter for each language separately:
+
+```json
+// ❌ This will NOT work
+{
+  "[javascript][typescript]": {
+    "editor.defaultFormatter": "esbenp.prettier-vscode"
+  }
+}
+
+// ✅ Use separate blocks instead
+{
+  "[javascript]": {
+    "editor.defaultFormatter": "esbenp.prettier-vscode"
+  },
+  "[typescript]": {
+    "editor.defaultFormatter": "esbenp.prettier-vscode"
+  }
+}
+```
+
 If you want to disable Prettier on a particular language you can either create a `.prettierignore` file or you can use VS Code's `editor.defaultFormatter` settings.
 
 The following will use Prettier for all languages except Javascript.

docs/troubleshooting.md

@@ -31,6 +31,35 @@ Or add to your `settings.json`:
 }
 ```
 
+### Combined Language Settings Not Working
+
+**Note:** VS Code does **NOT** support combined language settings for `editor.defaultFormatter`. While VS Code 1.63+ supports combining multiple languages for some settings using syntax like `[javascript][typescript]`, this feature does not work for the `editor.defaultFormatter` setting.
+
+❌ **This will NOT work:**
+
+```json
+{
+  "[markdown][yaml]": {
+    "editor.defaultFormatter": "esbenp.prettier-vscode"
+  }
+}
+```
+
+✅ **Instead, use separate language blocks:**
+
+```json
+{
+  "[markdown]": {
+    "editor.defaultFormatter": "esbenp.prettier-vscode"
+  },
+  "[yaml]": {
+    "editor.defaultFormatter": "esbenp.prettier-vscode"
+  }
+}
+```
+
+This is a VS Code core limitation, not an issue with the Prettier extension. The combined syntax works for other editor settings (like `editor.wordWrap`, `editor.tabSize`, etc.) but not for `editor.defaultFormatter`.
+
 ### Check for Errors in the Output Panel
 
 1. Click "Prettier" in the VS Code status bar (bottom right)