Plone 4 - Add valid_tags to the safe_html

metadata.xml

<?xml version="1.0"?>
<metadata>
    <version>002</version>
</metadata>

upgrades.py

from plone.app.controlpanel.filter import FilterControlPanelAdapter
from Products.CMFCore.utils import getToolByName


def html_filtering(context=None):
    """Change the HTML Filtering settings
    """
    fcpa = FilterControlPanelAdapter(context)
    transform = getattr(getToolByName(context, 'portal_transforms'), 'safe_html')
    new_tags = ['button', 'script', 'form']  # include any others here
    nasty_tags = dict(transform.get_parameter_value('nasty_tags'))
    valid_tags = dict(transform.get_parameter_value('valid_tags'))
    for tag in new_tags:
        if tag in valid_tags:
            continue
        if tag in nasty_tags:
            del nasty_tags[tag]
        valid_tags[tag] = 1
    fcpa._settransform(valid_tags=valid_tags)
    fcpa._settransform(nasty_tags=nasty_tags)

upgrades.zcml

<configure
    xmlns="http://namespaces.zope.org/zope"
    xmlns:genericsetup="http://namespaces.zope.org/genericsetup"
    i18n_domain="project.policy">

  <genericsetup:upgradeStep
      source="001"
      destination="002"
      sortkey="10"
      title="Change HTML Filter settings"
      description=""
      profile="project.policy:default"
      handler="project.policy.upgrades.html_filtering"
      />

</configure>

The version in metadata.xml should increment from whatever you have there now. The old and new values should match what you put into the zcml source and destination.

Don't remove any other code that may already be in these files.