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

[le0pard]

Motivation / Background

Mail preview support show inline attachments for <img> tag, but have not support for background-image css attribute. But this is also valid way to use inline images in emails. More info: https://email2go.io/blog/how-to-use-background-images-in-email-templates

Example of usage in erb template:

<td style="width: 100%; background-image: url(<%= attachments['welcome-email-bg.png'].url %>)">

without this fix:

with this fix:

Did not found tests for InlinePreviewInterceptor, so if I am missed add test cases, please point me where to add its