fix caption for attachments when they are set

ts/session/utils/Attachments.ts

@@ -75,6 +75,7 @@ export class AttachmentUtils {
       size: attachment.size,
       fileName: attachment.fileName,
       flags: attachment.flags,
+      caption: attachment.caption,
     };
 
     let attachmentData: ArrayBuffer;

ts/types/Attachment.ts

@@ -250,6 +250,7 @@ export function getAlt(
 
 export type Attachment = {
   fileName?: string;
+  caption?: string;
   flags?: SignalService.AttachmentPointer.Flags;
   contentType?: MIME.MIMEType;
   size?: number;