It appears Reset to Default on the Preferences page does not reset the Email Style. If you set it to HTML Mail with Attachments, then Workflow can send mail properly.
thank you for your reply.
But, what i'd like to do is to control initial value which will shown in of preference of user who never register preference.
Is there any idea?
When I create a new user in 12.1, I get HTML Mail with Attachments for default email style. Do you want something different to happen?