Recommended way to upload a document (work permit, visa, passport scan) when your network blocks direct S3 uploads (e.g. LNA / CASB) — the file streams through the API server, so only the API host needs to be reachable. The API rejects anything over 25 MB, but the deployed API runs behind AWS Lambda, whose request payload limit puts the practical ceiling far lower: our own dashboard switches to the chunked flow above 3.5 MB, and you should too. For anything larger use POST /users/file/upload-chunked/init → /chunk → /complete, which uploads in 4 MB parts. Attaching the file to a profile field is a SECOND step: this returns { id }, and you then send that id as the value of the field key via POST /users/{id}/data (for a FILES field, pass a comma-separated list of ids). Passing userFieldId here only validates the extension against that field’s allowed types — it does not create the link.