editor: File extension-based language detection.

core/client.js

@@ -1601,6 +1601,25 @@ function connectSocket(path) {
 	}
 }
 
+/**
+ * Determine a CodeMirror language mode from filename.
+ * @param name Filename.
+ * @return The mode name.
+ */
+function modeFromName(name) {
+	switch (name.split('.').pop()) {
+		case 'md':
+			return 'markdown';
+		case 'css':
+			return 'css';
+		case 'js':
+			return 'javascript';
+		case 'xml':
+		case 'svg':
+			return 'xml';
+	}
+}
+
 /**
  * Open a file by name.
  * @param name The file to open.
@@ -1612,6 +1631,7 @@ function openFile(name) {
 			: cm6.EditorState.create({doc: '', extensions: cm6.extensions});
 	let oldDoc = g_editor.state;
 	g_editor.setState(newDoc);
+	cm6.setEditorMode(g_editor, modeFromName(name));
 
 	if (g_files[g_current_file]) {
 		g_files[g_current_file].doc = oldDoc;