Drop group messages that don't change group

ts/util/downloadAttachment.ts

@@ -0,0 +1,32 @@
+import {
+  AttachmentPointerClass,
+  DownloadAttachmentType,
+} from '../textsecure.d';
+
+type AttachmentData = AttachmentPointerClass & {
+  id?: string;
+};
+
+export async function downloadAttachment(
+  attachmentData: AttachmentData
+): Promise<DownloadAttachmentType | null> {
+  let downloaded;
+  try {
+    if (attachmentData.id) {
+      // eslint-disable-next-line no-param-reassign
+      attachmentData.cdnId = attachmentData.id;
+    }
+    downloaded = await window.textsecure.messageReceiver.downloadAttachment(
+      attachmentData
+    );
+  } catch (error) {
+    // Attachments on the server expire after 30 days, then start returning 404
+    if (error && error.code === 404) {
+      return null;
+    }
+
+    throw error;
+  }
+
+  return downloaded;
+}