If you place the wysiwyg in the same "horizontal tab" (http://drupal.org/project/field_group) as the file field widget or on an edit for without any horizontal tab items it works as designed.
If you place the wysiwyg on Horiz tab 1 and the file widget on horiz tab 2 when you try to insert you get the following error and the action doesnt work:
E is undefined
It seems like the display: none; that horizontal tabs applies to the fieldset makes the wysiwyg unavailable for manipulation? Wontfix is a totally valid answer here but i thought you might have an idea that i hadnt considered for an easy fix. Cheers.