For multi-part upload the metadata must be sent during the initiation step: http://docs.aws.amazon.com/AmazonS3/latest/dev/mpuoverview.html
"If you want to provide any metadata describing the object being uploaded, you must provide it in the request to initiate multipart upload."

Without this the content-type of any file uploaded with multi-part upload is set to text/plain (the default value used in WSRequestHolder.prepare).