module ActionDispatch::Http::MimeNegotiation

GET /posts/5 | request.format => Mime[:html] or Mime[:js], or request.accepts.first
GET /posts/5.xhtml | request.format => Mime[:html]
GET /posts/5.xml | request.format => Mime[:xml]

Returns the MIME type for the \format used in the request.

def format(view_path = [])
  formats.first || Mime::NullType.instance
end