Enable browser caching for cloud files

[boxfoot]

Is your feature request related to a problem? Please describe. I use cloud file attachments, including some that are pretty large and take 10-50 seconds to download. Every time I reload my Observable page, these files are re-downloaded which can cause a pretty big delay and interrupts my work.

Describe the solution you'd like It would be great if Observable could leverage browser caching so that the file would not need to be re-downloaded. I noticed that there's already some indirection here where observable first requests a token and then uses that token to download the file. It would be great if somewhere in there it could detect when the cloud file was last updated and use e-tags or another caching mechanism to allow the browser to reuse the already-downloaded file.

Describe alternatives you've considered

Additional context

Thank you!