diff --git a/src/vs/editor/common/services/languagesRegistry.ts b/src/vs/editor/common/services/languagesRegistry.ts
index 3275621b978..c734735a733 100644
--- a/src/vs/editor/common/services/languagesRegistry.ts
+++ b/src/vs/editor/common/services/languagesRegistry.ts
@@ -153,9 +153,14 @@ export class LanguagesRegistry extends Disposable {
 		}
 
 		if (Array.isArray(lang.extensions)) {
+			if (lang.configuration) {
+				// insert first as this appears to be the 'primary' language definition
+				resolvedLanguage.extensions = lang.extensions.concat(resolvedLanguage.extensions);
+			} else {
+				resolvedLanguage.extensions = resolvedLanguage.extensions.concat(lang.extensions);
+			}
 			for (let extension of lang.extensions) {
 				mime.registerTextMime({ id: langId, mime: primaryMime, extension: extension }, this._warnOnOverwrite);
-				resolvedLanguage.extensions.push(extension);
 			}
 		}
 
diff --git a/src/vs/editor/test/common/services/languagesRegistry.test.ts b/src/vs/editor/test/common/services/languagesRegistry.test.ts
index 45c3ba17966..09ef74cd000 100644
--- a/src/vs/editor/test/common/services/languagesRegistry.test.ts
+++ b/src/vs/editor/test/common/services/languagesRegistry.test.ts
@@ -221,6 +221,32 @@ suite('LanguagesRegistry', () => {
 		assert.deepEqual(registry.getExtensions('aName'), ['aExt', 'aExt2']);
 	});
 
+	test('extensions of primary language registration come first', () => {
+		let registry = new LanguagesRegistry(false);
+
+		registry._registerLanguages([{
+			id: 'a',
+			extensions: ['aExt3']
+		}]);
+
+		assert.deepEqual(registry.getExtensions('a')[0], 'aExt3');
+
+		registry._registerLanguages([{
+			id: 'a',
+			configuration: URI.file('conf.json'),
+			extensions: ['aExt']
+		}]);
+
+		assert.deepEqual(registry.getExtensions('a')[0], 'aExt');
+
+		registry._registerLanguages([{
+			id: 'a',
+			extensions: ['aExt2']
+		}]);
+
+		assert.deepEqual(registry.getExtensions('a')[0], 'aExt');
+	});
+
 	test('filenames', () => {
 		let registry = new LanguagesRegistry(false);