Steps to reproduce the issue

Add debug code to function onContentPrepare in a plugin.
Display an article on site - event triggered OK
Display article edit form on administrator - event not triggered

Expected result

Expect onContentPrepare event to always be triggered in both site and administrator.

Actual result

onContentPrepare event not triggered in administrator (article editor)

System information (as much as possible)

Joomla 3.8.4

Additional comments

I want the change the <form ...> parameters from an event field plugin.
In particular I want to add enctype="multipart/form-data" .
I could do this with template overrides, but want to avoid touching them.

Looking at articles.php in administrator/components/com_content/views there appears to be no feature which would enable this (or other parameters) to be added the form without using the onContentPrepare event (on a field plugin). Rather than using the event to change the html produced it would be preferable if the field plugin could modify the form object to request this using another event (e.g. onContentPrepareForm).