ActionText::ContentHelper.allowed_attributes ||= []
ActionText::ContentHelper.allowed_attributes += [
  "target", # We set target="_blank" on external links
  "abbr",
  "alt",
  "cite",
  "class",
  "datetime",
  "height",
  "href",
  "lang",
  "name",
  "src",
  "title",
  "width",
  "xml:lang",
]
ActionText::ContentHelper.allowed_attributes += ActionText::Attachment::ATTRIBUTES