build: fix web_worker images example not working (#28562)

The web_worker images example is currently not really usable
because the rendered button that can be used to upload
an "image" to the demo is currently not working. This is because
the HTML markup for the `file-field` is not matching what `materialize-css`
expects. See: https://materializecss.com/text-inputs.html

PR Close #28562

modules/playground/src/web_workers/images/image_demo.html

@@ -24,10 +24,13 @@
     </a>
   </div>
 
-  <div class="file-field">
+  <div class="input-field file-field">
     <div class="btn blue darken-2">
       <span>Select Images</span>
       <input type="file" accept="image/bmp" multiple (change)="uploadFiles($event.target.files)" />
     </div>
+    <div class="file-path-wrapper">
+      <input class="file-path validate" type="text" placeholder="Upload one or more files">
+    </div>
   </div>
 </section>