Fixes #756 (#763)

apps/gateway/src/chat/chat.dto.ts

@@ -1,3 +1,4 @@
+import type { ChannelAttachmentDto } from '@mosaicstack/types';
 import { IsOptional, IsString, IsUUID, MaxLength } from 'class-validator';
 
 export class ChatRequestDto {
@@ -32,4 +33,7 @@ export class ChatSocketMessageDto {
   @IsOptional()
   @IsUUID()
   agentId?: string;
+
+  /** Validated channel attachment references; binary content is not embedded. */
+  attachments?: readonly ChannelAttachmentDto[];
 }