A presigned link to a package's file (a package is one file). For a
folder, request an archive.

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).

short: true returns a short link (/l/{token}, about 60 characters)
in place of the presigned one, with the same lifetime, for a client
that can't carry a presigned link: Microsoft's Office viewer caps
the URL it opens. Following it redirects to a presigned link signed
for 5 minutes, as often as needed while it lasts.