@thet @mauritsvanrees Thanks for the reply.
I think this is a bug, because this functionality was always available until version 5.1.x. and this code could helped fix that.
And taking a look at version 5.2 it seems that there are a couple of complications with this too (TinyMCE) List of TinyMCE defects in Plone 5.2 / call for fixing issues and participation
We wanted to have this for this version available because we have third party packages that have not been migrated to python 3
I respect your decision.
And I appreciate your latest helpful comments.