Major Description
Description of the issue is in this forum post: http://cxf.547215.n5.nabble.com/Setting-soap-attachment-Content-Transfer-Encoding-td3335835.html
Attachment encoding in CXF doesn't support base64. AttachmentSerializer.class in cxf-api contains the hardcoded value writer.write(URLDecoder.decode(attachmentId, "UTF-8"));