The following property is added to the configuration file filestore-s3.properties:

# Optionally configures an override for the algorithm used to sign requests
# against the S3 interface. If left empty, the underlying client will choose a
# suitable signer type based on the actually used service and region
# automatically, however, auto-detection only works properly for the vanilla
# Amazon S3, and may lead to unwanted results when targeting an S3
# implementation where not all signature types are supported.
# Possible values include S3SignerType to enforce the AWS signature v2, and
# AWSS3V4SignerType for AWS signature v4.
# For backwards compatibility, the setting defaults to S3SignerType, and
# needs to be adjusted explicitly when targeting the newer v4-only regions
# like Frankfurt or Beijing.
com.openexchange.filestore.s3.[filestoreID].signerOverride=S3SignerType

Additionally, the comments for property com.openexchange.filestore.s3.[filestoreID].region have been adjusted to include the newly supported regions:

# Configures the Amazon S3 region to use when creating new buckets. This value
# is also used to pre-configure the client when no specific endpoint is set.
# Possible values are us-gov-west-1, us-east-1, us-west-1, us-west-2,
# eu-west-1, eu-central-1, ap-southeast-1, ap-southeast-2,
# ap-northeast-1, sa-east-1, cn-north-1. Defaults to us-west-2.
com.openexchange.filestore.s3.[filestoreID].region=us-west-2