This is called "full" and "simple" editor type.
You just need to specify it to 'input/longtext' in the entity create/edit form.
For example, for full type you don't need to specify anything, but for simple type you do.
I don't know anything about Links and Downloads, but for Discussions you should override the create discussion form in your custom plugin and remove this line:
'editor_type' => 'simple',
info@elgg.org
Security issues should be reported to security@elgg.org!
©2014 the Elgg Foundation
Elgg is a registered trademark of Thematic Networks.
Cover image by RaĆ¼l Utrera is used under Creative Commons license.
Icons by Flaticon and FontAwesome.