Whether returned file is from cache or just downloaded

## 💬 Questions and Help

Hi, is there an easy way to check during `.getSingleFile()` whether the returned file was pulled from the cache, or whether it was just downloaded?
Thank you!

从 getSingleFile() 入口点开始，检查缓存查找和下载路径，以确定现有 API 是否公开返回文件的来源。明确预期的可观测结果，并通过一个针对性测试覆盖该结果；当用户能够可靠地区分缓存文件和新下载的文件时，即表示完成。

由索引模型根据 Issue 内容生成。