#11558: Better message if attach called on non-multipart.

Original patch by Varun Sharma.
diff --git a/Misc/ACKS b/Misc/ACKS
index dbf83e6..9c559ec 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1188,6 +1188,7 @@
 Ha Shao
 Mark Shannon
 Richard Shapiro
+Varun Sharma
 Vlad Shcherbina
 Justin Sheehy
 Charlie Shepherd