# File lib/action_mailer/base.rb, line 671 def set_content_type(m, user_content_type, class_default) params = m.content_type_parameters || {} case when user_content_type.present? user_content_type when m.has_attachments? if m.attachments.detect { |a| a.inline? } ["multipart", "related", params] else ["multipart", "mixed", params] end when m.multipart? ["multipart", "alternative", params] else m.content_type || class_default end end