Tell Products.TinyMCE what the MIME type is, so it doesn't have to work it out.
Force WYSIWYG for Products.TinyMCE

This means that when we start with 'text/plain' (or another MIME),
selecting 'text/html' will cause TinyMCE to spring into life. It won't
override the user's editor preference.
Merge pull request #12 from lentinj/master

Fixes to improve type handling for plone.app.textfield in Lists