FreeScout's client-controlled attachment IDs allow deletion of existing conversation attachments

FreeScout is a free self-hosted help desk and shared mailbox. Prior to version 1.8.215, the reply and draft flows trust client-supplied encrypted attachment IDs. Any IDs present in `attachments_all[]` but omitted from retained lists are decrypted and passed directly to `Attachment::deleteByIds()`. Because `load_attachments` returns encrypted IDs for attachments on a visible conversation, a mailbox peer can replay those IDs through `save_draft` and delete the original attachment row and file. Version 1.8.215 fixes the vulnerability.