Creates a secure one-time upload link (valid 30 minutes) for the fax document (PDF, JPG or PNG, max. 10 MB) – upload from phone or computer by drag & drop, file picker or camera. For files on the user's device, or chat attachments that are not available as send_fax's "file" parameter. The user opens upload_url and uploads the file; send_fax then accepts the returned upload_id. Creating a link sends nothing and costs nothing.