On a Plone 5.0.10 site that I still have to upgrade, I found that TinyMCE uses a predefined list of allowed image scales, which does not take into account custom scales defined in the control panel.
The bug was fixed many years ago with the release of Mockup 2.3.0 and made it into Plone 5.1.
After putzing around for hours: trying to shoehorn plone.staticresources and a new Mockup in 5.0 while recompiling js resources, removing duplicate zcml directives and more of that fun, I decided that there were too many moving parts... I then went back to a small PR from @pbauer which had been rejected in favor of the Mockup fixes.
With only a few minor changes to Philip's code, I was able to fix my issue. The get_scales() function goes in
./patterns/__init__.py and the IImagingSchema is imported
from Products.CMFPlone.interfaces.controlpanel import IImagingSchema
I hope this helps someone... Related posts on this forum: