Enable syntax highlighting for new files

Dispatch native 'change' event instead of jQuery when
the filename gets changed

Changelog: fixed

app/assets/javascripts/blob/file_template_mediator.js

@@ -247,7 +247,11 @@ export default class FileTemplateMediator {
   }
 
   setFilename(name) {
-    this.$filenameInput.val(name).trigger('change');
+    const input = this.$filenameInput.get(0);
+    if (name !== undefined && input.value !== name) {
+      input.value = name;
+      input.dispatchEvent(new Event('change'));
+    }
   }
 
   getSelected() {

spec/frontend/blob/file_template_mediator_spec.js

+import TemplateSelectorMediator from '~/blob/file_template_mediator';
+
+describe('Template Selector Mediator', () => {
+  let mediator;
+
+  describe('setFilename', () => {
+    let input;
+    const newFileName = 'foo';
+    const editor = jest.fn().mockImplementationOnce(() => ({
+      getValue: jest.fn().mockImplementation(() => {}),
+    }))();
+
+    beforeEach(() => {
+      setFixtures('<div class="file-editor"><input class="js-file-path-name-input" /></div>');
+      input = document.querySelector('.js-file-path-name-input');
+      mediator = new TemplateSelectorMediator({
+        editor,
+        currentAction: jest.fn(),
+        projectId: jest.fn(),
+      });
+    });
+
+    it('fills out the input field', () => {
+      expect(input.value).toBe('');
+      mediator.setFilename(newFileName);
+      expect(input.value).toBe(newFileName);
+    });
+
+    it.each`
+      name           | newName        | shouldDispatch
+      ${newFileName} | ${newFileName} | ${false}
+      ${newFileName} | ${''}          | ${true}
+      ${newFileName} | ${undefined}   | ${false}
+      ${''}          | ${''}          | ${false}
+      ${''}          | ${newFileName} | ${true}
+      ${''}          | ${undefined}   | ${false}
+    `(
+      'correctly reacts to the name change when current name is $name and newName is $newName',
+      ({ name, newName, shouldDispatch }) => {
+        input.value = name;
+        const eventHandler = jest.fn();
+        input.addEventListener('change', eventHandler);
+
+        mediator.setFilename(newName);
+        if (shouldDispatch) {
+          expect(eventHandler).toHaveBeenCalledTimes(1);
+        } else {
+          expect(eventHandler).not.toHaveBeenCalled();
+        }
+      },
+    );
+  });
+});