FEATURE: new 'should_add_email_attachments' plugin modifier

That can be used by plugins to control whether email attachments should be sent.

Internal ref - t/132149

lib/email/sender.rb

@@ -360,6 +360,8 @@ module Email
 
       email_size = 0
       posts.each do |post|
+        next unless DiscoursePluginRegistry.apply_modifier(:should_add_email_attachments, post)
+
         post.uploads.each do |original_upload|
           optimized_1X = original_upload.optimized_images.first