The decisive fact arrived: the images DO render in ChatGPT. So a valid
signature exists for files that hand us 403, and there is a route to find.
The rest is now pinned down. /files/{id}/download is the minting endpoint —
a working file's download_url is the same estuary/content URL the browser
uses. Signatures are per-file: swapping an id into a working URL returns
"Invalid signature or expired URL", and a ts without a sig gets the same.
So the difference is in the request, not the route.
Our call sends Authorization: Bearer plus cookies. A browser leans on
cookies and adds client headers a gated endpoint may check. This walks
those variants — no Authorization, conversation Referer, oai-device-id,
oai-language, sec-fetch-*, an image Accept, a conversation_id param —
against a file that is currently refused, and reports which mints a URL.
If none do, it prints the DevTools recipe for finding the minting call by
searching the Network panel for the file id.
