Fixes originally added as part of CONF-2795 ensured uploading a new version of an attachment with a different case creates a new version of that attachment but does not change its original filename.

The RPC method addAttachment doesn't respect this. It has its own implementation which calls the AttachmentManager directly, rather than using the DefaultFileUploadManager like the attachment actions do.

The code for the remote API should use the same mechanism and have the same constraints as the attachment actions.