A presigned link to one file of a published, open version. paths
names exactly one file; a folder is refused. Embargoed versions need
signing in (the Downloads API).

purpose: download (the default) saves the file; the link lasts 15
minutes. purpose: view opens it in the browser for a viewer; the
link lasts an hour, and it is recorded as a view, never a download.
A view is served as the type its extension names (a PDF, an image, a
video, CSV), whatever type it was stored with; HTML, SVG, XML and
JavaScript are served as stored.
purpose: handoff is a download for another service to fetch (e.g.
oSPARC): it is saved like a download and counted as one, and the
link lasts as long as the agent's manifest links (3 hours; 1 hour on
external buckets).
Either way the file's size counts against the day's anonymous
allowance.