Pre-upload credentials and S3 direct-upload parameters
| Name | Type | Description | Notes |
|---|---|---|---|
| file_key | str | Base64 temporary object path; pass back unchanged on business submit—do not decode | |
| url | str | S3 direct upload URL | |
| fields | OtcUploadPreUploadPolicyFields | ||
| expires_in | int | Policy validity period in seconds; currently 5400 (90 minutes); aligns with `expiration` in `fields.Policy`; call this endpoint again after expiry |