[Minor] Increase scoring of *.exe attachments

These are already banned in most environments, including GMail and
Outlook. We may as well bump this score up to 4 eventually.

src/plugins/lua/mime_types.lua

@@ -53,7 +53,7 @@ local settings = {
 
   bad_extensions = {
     cue = 2,
-    exe = 1,
+    exe = 2,
     iso = 4,
     jar = 2,
     -- In contrast to HTML MIME parts, dedicated HTML attachments are considered harmful