Your configuration is passed down to the provider. (e.g: new AWS.S3(config)). You can see the complete list of options here
Example
./extensions/upload/config/settings.json
{
"provider": "aws-s3-webp-optimized",
"providerOptions": {
"accessKeyId": "dev-key",
"secretAccessKey": "dev-secret",
"region": "aws-region",
"params": {
"Bucket": "my-bucket"
}
}
}