public function updateCMSFields(FieldList $fields)
 {
     if (Config::inst()->get('MarkdownExtension', 'replace_html_fields')) {
         foreach ($fields->dataFields() as $field) {
             if ($field instanceof HtmlEditorField) {
                 $attributes = $field->getAttributes();
                 $fields->replaceField($field->getName(), MarkdownEditorField::create($field->getName(), $field->Title())->setRows($attributes['rows']));
             }
         }
     }
 }
<?php

define('MARKDOWN_MODULE_BASE', basename(dirname(__FILE__)));
MarkdownExtension::ReplaceHTMLFields();
MarkdownEditorField::include_default_js();
if (ClassInfo::exists('CloudinaryUploadField')) {
    MarkdownEditorField::add_extension('MarkdownCloudinaryUpload');
}
Object::useCustomClass('HtmlEditorField_Toolbar', 'MarkdownEditorField_Toolbar', true);