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
@@ -118,6 +118,7 @@
   viewer: Viewer,
   request: SendMultimediaMessageRequest,
 ): Promise<SendMessageResponse> {
+  invariant(request.mediaIDs, 'request.mediaIDs should be set');
   const { threadID, localID, mediaIDs } = request;
   if (mediaIDs.length === 0) {
     throw new ServerError('invalid_parameters');
diff --git a/lib/types/message-types.js b/lib/types/message-types.js
--- a/lib/types/message-types.js
+++ b/lib/types/message-types.js
@@ -67,6 +67,7 @@
 import type {
   MediaMessageData,
   MediaMessageInfo,
+  MediaMessageServerDBContent,
   RawMediaMessageInfo,
 } from './messages/media';
 import type {
@@ -499,11 +500,17 @@
   +localID?: string,
   +text: string,
 };
-export type SendMultimediaMessageRequest = {
-  +threadID: string,
-  +localID: string,
-  +mediaIDs: $ReadOnlyArray<string>,
-};
+export type SendMultimediaMessageRequest =
+  | {
+      +threadID: string,
+      +localID: string,
+      +mediaIDs: $ReadOnlyArray<string>,
+    }
+  | {
+      +threadID: string,
+      +localID: string,
+      +mediaMessageContents: $ReadOnlyArray<MediaMessageServerDBContent>,
+    };
 
 // Used for the message info included in log-in type actions
 export type GenericMessagesResult = {