Devise Invitable mailer -- how to force text/plain e-mail?

I am using Rails 3.09 and the latest Devise and Devise Invitable. I am able to make Devise send confirmation messages as text/plain just by adding a view properly named to the views/devise/mailer folder. Invitable keeps its views in views/users/invitations/mailer, but changing the name of the file (and the contents) does not seem to affect this at all. Instead, the view is being loaded out of the gem path and is the original HTML format.

I've looked through the source of invitable, and haven't found any configuration options for this. Does anyone have an idea how I can force the issue? I had all this worked out in 2.3, but I'm trying to get it to work in 3.0 now.

Thanks,

Walter