Notify when attachment download succeeds, fix deadlock in attachment store

kordophoned/src/daemon/mod.rs

@@ -314,6 +314,16 @@ impl Daemon {
 
                 reply.send(()).unwrap();
             }
+
+            Event::AttachmentDownloaded(attachment_id) => {
+                log::info!(target: target::ATTACHMENTS, "Daemon: attachment downloaded: {}, sending signal", attachment_id);
+
+                // Send signal to the client that the attachment has been downloaded.
+                self.signal_sender
+                    .send(Signal::AttachmentDownloaded(attachment_id))
+                    .await
+                    .unwrap();
+            }
         }
     }