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!

Start at the getSingleFile() entry point and inspect the cache lookup and download paths to determine whether the existing API exposes where the returned file came from. Establish the intended observable result and cover it with a focused test; done means users can reliably distinguish cached files from newly downloaded files.

索引モデルが issue の本文から書いたものです。