I've just converted my Rails 4 app from Haml to Slim. Everything went well but I can't get plain text emails to work.
I used the haml2slim converter. It automatically changed my filenames to filename.text.plain.slim.
When I run my tests, each one related to Mailers fails with an "ActionView::MissingTemplate" error.
I think your template name should be filename.text.slim
instead of filename.text.plain.slim
.