This binds that to a class name.  The extension point should have a contenttype binding id, so we say "which content type the description helper helps with".  And then here we can get the file that is being worked on, the resource, and then get its content type.  resource.getContentType() and then look through all the helpers and find the helper that is a helper for that content type.  So this way the lookup mechanism isn't tied to a classname.