为什么ActionMailer不随机显示我的多部分/替代邮件的纯文本部分?

I'm trying to send emails that have both a plain text and html component. There are no attachments, but there are inline images in the HTML part. I'm using the content-type "multipart/alternative". I'm not explicitly setting the parts; I'm relying on the filenames to determine the parts. I'm on rails 2.3.10.I'm trying to send emails that have both a plai