Issue/14435 multipart mail preview

MailPreviews with inline attachments error out. They try to render an HTML or
plain text part, but it only sees the attachment part (`image/png` for
example) and a `multipart/relative`, returning an unexpected array when views
expect a `Message::Part`.

If email is of type 'multipart/relative', then we render the `text/html` part
contained in it.

We'd like to render the image using a URL like `http://localhost:3000/rails/mailers/dog_mailer/dog_view?part=image/png`, anyway submitting for review.

Fix for issue: https://github.com/rails/rails/issues/14435.

Pairing with @masonforest. cc @pixeltrix @sikachu.