1 files changed, 49 insertions, 1 deletions
diff --git a/actionmailer/lib/action_mailer/preview.rb b/actionmailer/lib/action_mailer/preview.rb
index 44cf6665ba..4888eac345 100644
--- a/actionmailer/lib/action_mailer/preview.rb
+++ b/actionmailer/lib/action_mailer/preview.rb
@@ -1,9 +1,57 @@
 require 'active_support/descendants_tracker'
+require 'base64'
 
 module ActionMailer
   module Previews #:nodoc:
     extend ActiveSupport::Concern
 
+    class InlineAttachments #:nodoc:
+      PATTERN  = /src=(?:"cid:[^"]+"|'cid:[^']+')/i
+
+      include Base64
+
+      attr_reader :message
+
+      def self.previewing_email(message)
+        new(message).transform!
+      end
+
+      def initialize(message)
+        @message = message
+      end
+
+      def transform!
+        return message if html_part.blank?
+
+        html_source.gsub!(PATTERN) do |match|
+          if part = find_part(match[9..-2])
+            %[src="#{data_url(part)}"]
+          else
+            match
+          end
+        end
+
+        message
+      end
+
+      private
+        def html_part
+          @html_part ||= message.html_part
+        end
+
+        def html_source
+          html_part.body.raw_source
+        end
+
+        def data_url(part)
+          "data:#{part.mime_type};base64,#{urlsafe_encode64(part.body.raw_source)}"
+        end
+
+        def find_part(cid)
+          message.all_parts.find{ |p| p.attachment? && p.cid == cid }
+        end
+    end
+
     included do
       # Set the location of mailer previews through app configuration:
       #
@@ -21,7 +69,7 @@ module ActionMailer
 
       # :nodoc:
       mattr_accessor :preview_interceptors, instance_writer: false
-      self.preview_interceptors = []
+      self.preview_interceptors = [ActionMailer::Previews::InlineAttachments]
     end
 
     module ClassMethods