feat: add examples: form-upload (#5955)

* feat: add examples: form-upload

* fix: upload: accept and label

* fix: upload: 设置表单值、图片预览

apps/backend-mock/routes/[...].ts

@@ -7,6 +7,7 @@ export default defineEventHandler(() => {
 <li><a href="/api/menu">/api/menu/all</a></li>
 <li><a href="/api/auth/codes">/api/auth/codes</a></li>
 <li><a href="/api/auth/login">/api/auth/login</a></li>
+<li><a href="/api/upload">/api/upload</a></li>
 </ul>
 `;
 });