if as_attachment is in the url, add it to the sign_url (#18930)

api/models/model.py

                 sign_url = file_helpers.get_signed_file_url(upload_file_id)
             else:
                 continue
-
+            # if as_attachment is in the url, add it to the sign_url.
+            if "as_attachment" in url:
+                sign_url += "&as_attachment=true"
             re_sign_file_url_answer = re_sign_file_url_answer.replace(url, sign_url)
 
         return re_sign_file_url_answer