The S3 arm, shared by 's3' and the 'b2' (Backblaze) preset, which reaches
the same provider over B2's S3-compatible endpoint.
readonly
provider: "s3" | "b2"
Selects the S3 backend, or the Backblaze B2 preset over it.
readonly
options: S3ProviderOptions
Bucket and credentials; bucket is required.