if their content type ended in xml ( such as image/svg+xml ),
TextDocument._convertToBaseFormat was resetting the content type to
application/xml.

When doing something like:

  wp = web_page_module.newContent(
    portal_type='Web Page',
    content_type='image/svg+xml',
    text_content=svg
  )

wp.getContentType() was sometimes 'image/svg+xml' sometimes
'application/xml', depending on the order of edit.