better d-bus interface for attachments

kordophoned/src/daemon/events.rs

@@ -62,6 +62,12 @@ pub enum Event {
     /// - reply: Reply of the attachment object, if known.
     GetAttachment(String, Reply<Attachment>),
 
+    /// Downloads an attachment from the server.
+    /// Parameters:
+    /// - attachment_id: The attachment ID to download
+    /// - reply: Reply indicating success or failure
+    DownloadAttachment(String, Reply<()>),
+
     /// Delete all conversations from the database.
     DeleteAllConversations(Reply<()>),
 }