class Rails::Application::Configuration

A LoadError is raised if any of the specified extensions can't be found.

config.extensions = [:dashboard, :blog, :all, :comments]
config.extensions = [:dashboard, :blog, :all]
config.extensions = [:layouts, :taggable, :all]
It can include an :all marker to mean 'everything else' and is typically set in environment.rb:
Sets the list of extensions that will be loaded and the order in which to load them.

def extensions=(extensions)
  @requested_extensions = extensions
end