File Sharing and Storage for groups.
Quip API is a free Cloud Storage & File Sharing API that requires apiKey authentication, is served over HTTPS and supports CORS, so it can be called from the browser.
Details from quip.com.

Unified API suite · now live
Geocode an address, validate an email, track a flight or pull stock data with one account, one dashboard and one API key.
Free JSON storage for small projects
No-bullshit file hosting and URL shortening service
The Quip API is provided by Quip Automation API Reference (quip.com).
Yes. The Quip API is listed in the Cloud Storage & File Sharing category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
Yes. The Quip API uses apiKey authentication, so you need to register for credentials before making requests.
Yes. The Quip API supports CORS, so front-end JavaScript can call it directly.
Yes. The Quip API is served over HTTPS, so it's safe to use in production.