rails/rails — pull request #15569 by y-yagi (2014-06-08 06:58:47+00:00)
Title: Find mail part recursively

I'm getting a `NoMethodError in Rails::Mailers#preview` when I try to preview an email view that has a attachment.

More precisely:
```ruby
undefined method `mime_type' for #
```

ActionMailer generate a `multipart/alternative` email when use `attachments`. It is necessary to look for mail part recursively.