respond_to with MIME type wildcards?

I'd like to respond to requests for all kinds of image MIME types from a
single respond_to. In particular, I don't want to register MIME types
for each image format, I'd rather define only one with a wildcard, like
this

  Mime::Type.register 'image/*', :image

Then, in the controller handle it like this

  def show
    respond_to do |format|
      format.image do
        # check if concrete MIME type can be provided
        ...
        send_file @image.filename, :disposition => 'inline'
      end

      format.html do
        # send HTML page embedding the image
      end
    end
  end

As it happens, this doesn't work. Registered MIME types are matched
literally, the '*' is not interpreted as a wildcard.

The best I've come up with is this

  Mime::Type.register 'image/png', :png
  Mime::Type.register 'image/jpeg', :jpg, ['image/jpg'], [:jpeg]

and then

      format.any(:png, :jpg) do
        send_file @image.filename, :disposition => 'inline'
      end

Is there a another way?

Michael