diff --git a/keyserver/src/responders/message-responders.js b/keyserver/src/responders/message-responders.js
--- a/keyserver/src/responders/message-responders.js
+++ b/keyserver/src/responders/message-responders.js
@@ -118,8 +118,11 @@
   viewer: Viewer,
   request: SendMultimediaMessageRequest,
 ): Promise<SendMessageResponse> {
-  invariant(request.mediaIDs);
-  const { threadID, localID, mediaIDs } = request;
+  const { threadID, localID } = request;
+  const mediaIDs =
+    request.mediaIDs ??
+    request.mediaMessageContents.map(contents => contents.uploadID);
+
   if (mediaIDs.length === 0) {
     throw new ServerError('invalid_parameters');
   }