Get File Download Url

Fresh signed URL with a forced-download Content-Disposition header.

Distinct from the file's own preview/inline URL (surfaced elsewhere as previewUrl/downloadUrl on the file-list read) — that one is deliberately left WITHOUT a Content-Disposition so PDFs/images still render inline in the preview panel. A plain <a download> tag's download attribute is silently ignored by browsers for a cross-origin URL (an S3 signed URL always is), so reusing the inline URL for the "Download" button just navigated the tab to it — rendering the file inline, or dumping S3's raw NoSuchKey XML error page in place of a download when the underlying object is missing/stale (the exact bug this endpoint was added to fix, found live 2026-08-01). This endpoint is called fresh on every click rather than cached alongside the file list, since the URL is single-purpose and short-lived anyway.