Cloud file storage, version history, sharing and incoming email attachments.
Revdoku API is a free Cloud Storage & File Sharing API that requires apiKey authentication, is served over HTTPS and has unconfirmed CORS support.

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Yes. The Revdoku 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 Revdoku API uses apiKey authentication, so you need to register for credentials before making requests.
CORS support for the Revdoku API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The Revdoku API is served over HTTPS, so it's safe to use in production.