env variable holding a previously
downloaded path, then primes the file chooser so the next click that triggers a
file dialog uploads the file. For local CLI runs, use a path such as
fixtures/sample.pdf, resolved against the CLI working directory. Paths
beginning with ./ or ../ are interpreted as uploaded user-file references,
rather than files on disk. Use momentic://sample.pdf to reference an uploaded
user file explicitly.
For cloud runs, use a public URL, an uploaded user file, or a file downloaded
earlier in the same browser session.