The key for the S3 object that this file should be uploaded to.
The string can have placeholder expressions which are resolved at upload time. Valid expressions are strings that are valid Java DateTimeFormatter strings. See https://docs.oracle.com/javase/8/docs/api/java/time/format/DateTimeFormatter.html
User metadata. For key of a user metadata, callers should not include the internal "x-amz-meta-" prefix. Keys are case insensitive and will appear as lowercase strings on S3, even if they were originally specified with uppercase strings. Reserved key names start with "$aws-gg-" prefix.