4 files changed, 64 insertions, 2 deletions
diff --git a/actionmailer/lib/action_mailer/base.rb b/actionmailer/lib/action_mailer/base.rb
index 754f698e48..218b7a735a 100644
--- a/actionmailer/lib/action_mailer/base.rb
+++ b/actionmailer/lib/action_mailer/base.rb
@@ -596,6 +596,7 @@ module ActionMailer
 
     class NullMail #:nodoc:
       def body; '' end
+      def header; {} end
 
       def respond_to?(string, include_all=false)
         true
diff --git a/actionmailer/lib/action_mailer/inline_preview_interceptor.rb b/actionmailer/lib/action_mailer/inline_preview_interceptor.rb
new file mode 100644
index 0000000000..dea5e52f0c
--- /dev/null
+++ b/actionmailer/lib/action_mailer/inline_preview_interceptor.rb
@@ -0,0 +1,61 @@
+require 'base64'
+
+module ActionMailer
+  # Implements a mailer preview interceptor that converts image tag src attributes
+  # that use inline cid: style urls to data: style urls so that they are visible
+  # when previewing a HTML email in a web browser.
+  #
+  # This interceptor is enabled by default, to remove it delete it from the
+  # <tt>ActionMailer::Base.preview_interceptors</tt> array:
+  #
+  #   ActionMailer::Base.preview_interceptors.delete(ActionMailer::InlinePreviewInterceptor)
+  #
+  class InlinePreviewInterceptor
+    PATTERN  = /src=(?:"cid:[^"]+"|'cid:[^']+')/i
+
+    include Base64
+
+    def self.previewing_email(message) #:nodoc:
+      new(message).transform!
+    end
+
+    def initialize(message) #:nodoc:
+      @message = message
+    end
+
+    def transform! #:nodoc:
+      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 message
+        @message
+      end
+
+      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,#{strict_encode64(part.body.raw_source)}"
+      end
+
+      def find_part(cid)
+        message.all_parts.find{ |p| p.attachment? && p.cid == cid }
+      end
+  end
+end
diff --git a/actionmailer/lib/action_mailer/preview.rb b/actionmailer/lib/action_mailer/preview.rb
index 44cf6665ba..25ad7ee721 100644
--- a/actionmailer/lib/action_mailer/preview.rb
+++ b/actionmailer/lib/action_mailer/preview.rb
@@ -21,7 +21,7 @@ module ActionMailer
 
       # :nodoc:
       mattr_accessor :preview_interceptors, instance_writer: false
-      self.preview_interceptors = []
+      self.preview_interceptors = [ActionMailer::InlinePreviewInterceptor]
     end
 
     module ClassMethods
diff --git a/actionmailer/lib/action_mailer/test_helper.rb b/actionmailer/lib/action_mailer/test_helper.rb
index 524e6e3af1..4d03a616d2 100644
--- a/actionmailer/lib/action_mailer/test_helper.rb
+++ b/actionmailer/lib/action_mailer/test_helper.rb
@@ -34,7 +34,7 @@ module ActionMailer
         original_count = ActionMailer::Base.deliveries.size
         yield
         new_count = ActionMailer::Base.deliveries.size
-        assert_equal original_count + number, new_count, "#{number} emails expected, but #{new_count - original_count} were sent"
+        assert_equal number, new_count - original_count, "#{number} emails expected, but #{new_count - original_count} were sent"
       else
         assert_equal number, ActionMailer::Base.deliveries.size
       end