feat: Display file extensions in File Browser (#1019)

## Summary

![IMG_8136
Medium](https://github.com/user-attachments/assets/e33a8a71-4126-4bdc-8d3a-32132acba712)
![IMG_8137
Medium](https://github.com/user-attachments/assets/2289fbee-71e8-446c-ad28-9dd4a9a21396)
![IMG_8138
Medium](https://github.com/user-attachments/assets/0e912f72-9be1-4708-a909-4dab192d1aea)


## Additional Context

Do we want a setting to toggle this?

---

### AI Usage

Did you use AI tools to help write this code? _**NO**_

src/activities/home/FileBrowserActivity.cpp

@@ -249,6 +249,14 @@ std::string getFileName(std::string filename) {
   return filename.substr(0, pos);
 }
 
+std::string getFileExtension(std::string filename) {
+  if (filename.back() == '/') {
+    return "";
+  }
+  const auto pos = filename.rfind('.');
+  return filename.substr(pos);
+}
+
 void FileBrowserActivity::render(RenderLock&&) {
   renderer.clearScreen();
 
@@ -267,7 +275,8 @@ void FileBrowserActivity::render(RenderLock&&) {
     GUI.drawList(
         renderer, Rect{0, contentTop, pageWidth, contentHeight}, files.size(), selectorIndex,
         [this](int index) { return getFileName(files[index]); }, nullptr,
-        [this](int index) { return UITheme::getFileIcon(files[index]); });
+        [this](int index) { return UITheme::getFileIcon(files[index]); },
+        [this](int index) { return getFileExtension(files[index]); }, false);
   }
 
   // Help text