We have a DynamicContent plugin, which is a usercontrol inheriting from EPiServer.UserControlBase, and which is decorated with the DynamicContentPlugIn-attribute. It has a single public property:

public PropertyXhtmlString MainBody { get; set; }

When the editors try to create a new instance of this DynamicContent, the edit interface displays a simple textarea (as it would for a LongString-property), without the TinyMCE toolbar.

How can I fix this?