feat: Add file names to snippets

contents/upload-button.mdx

@@ -9,7 +9,7 @@ title: Upload button
 
 ## HTML
 
-```html
+```html index.html
 <div class="upload-button">
     <!-- The real file input -->
     <input type="file" class="upload-button__input" />
@@ -24,7 +24,7 @@ title: Upload button
 
 ## CSS
 
-```css
+```css styles.css
 .upload-button {
     /* Used to position the input */
     position: relative;