Support css background-image attributes with cid: on mail preview

actionmailer/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Support css background-image attributes with cid: on mail preview
+
+    *Alexey Vasiliev*
+
 *   Remove deprecated params via `:args` for `assert_enqueued_email_with`.
 
     *Rafael Mendonça França*

actionmailer/lib/action_mailer/inline_preview_interceptor.rb

@@ -6,16 +6,16 @@ module ActionMailer
   # = Action Mailer \InlinePreviewInterceptor
   #
   # 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 an HTML email in a web browser.
+  # or css attributes that use inline cid: style URLs to data: style URLs
+  # so that they are visible when previewing an HTML email in a web browser.
   #
   # This interceptor is enabled by default. To disable 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
+    PATTERN = /src=(?:"cid:[^"]+"|'cid:[^']+')|url\((?:"cid:[^"]+"|'cid:[^']+'|cid:[^\)]+)\)/i
 
     include Base64
 
@@ -31,8 +31,8 @@ def transform! # :nodoc:
       return message if html_part.blank?
 
       html_part.body = html_part.decoded.gsub(PATTERN) do |match|
-        if part = find_part(match[9..-2])
-          %[src="#{data_url(part)}"]
+        if part = find_part(match.match(/cid:([^"'\)]+)/i)[1])
+          match.start_with?('src=') ? %[src="#{data_url(part)}"] : %[url(#{data_url(part)})]
         else
           match
         end