muttrc: add attachments indication to index.

Signed-off-by: Andrey Drobyshev <drmtmych@gmail.com>

muttrc

@@ -13,7 +13,7 @@ set timeout = "5"
 set mail_check = "10"
 set mailcap_path = ~/.config/mutt/etc/mailcap
 set date_format="%m/%d %I:%M"
-set index_format="%2C %Z %D %-15.15F %s (%-4.4c)"
+set index_format="%2C %Z %?X?A& ? %D %-15.15F %s (%-4.4c)"
 set markers = no
 set mark_old = no
 set mime_forward = yes