docs: add section on testing providers (#9453)

www/apps/resources/references/file/classes/file.AbstractFileProviderService/page.mdx

@@ -230,3 +230,9 @@ module.exports = defineConfig({
   }
 })
 ```
+
+---
+
+## 5. Test it Out
+
+To test out your file provider, use the Medusa Admin or the [Upload API route](https://docs.medusajs.com/v2/api/admin#uploads_postuploads) to upload a file.