Inline Attachments don't seem to work

First of all, awesome gem. It's extremely helpful. It doesn't seem like it's able to display inline attachments, though.

In my mailer, I set an inline attachment:
```ruby
def welcome(user)
  attachments.inline['logo.png'] = File.read(Rails.root.join('app/assets/images/logo.png'))
  mail(to: user.email, subject: 'Example Email')
end
```

In my view I display the attachment:
```ruby
image_tag(attachments['logo.png'].url)
```

When I load up Rails Email Preview, I see this in the js console:
```
GET cid:537643989ad47_f68104fe889515@ray-desktop.mail net::ERR_UNKNOWN_URL_SCHEME 
```

When I send the email, everything works as expected, and the image displays.