Responding to a non-registered MIME type?

I'm trying to figure out how attachments are supposed to work with MIME
type-based responses. What makes attachments special is that in many
cases they are allowed to be arbitrary files. It would be too
restrictive to require that all possible types are registered
(Mime::Type.register).

Let's say for GET requests on an attachment resource I want to return
metadata if the requested format is XML or JSON. Otherwise I want to
return the attachment itself.

  class AttachmentsController < ApplicationController
    respond_to :xml, :json
    respond_to :all, :only => :show

    def show
      @attachment = ...
      respond_with(@attachment) do
        format.any { send_file @attachment.path }
      end
    end
  end

This doesn't work, because #any really means any, including :xml and
:json. This doesn't work as intended, either

      respond_with(@attachment) do
        format.xml
        format.json
        format.any { send_file @attachment.path }
      end

It has to be

      respond_with(@attachment) do
        format.xml {}
        format.json {}
        format.any { send_file @attachment.path }
      end

But that is not the kind of code I'd like to write. I'd rather write

      respond_with(@attachment) do
        format.default { send_file @attachment.path }
      end

With the intended meaning that the default block is only used if
respond_to :all is active for the action and there is no more specific
explicit or implicit response for the format.

Did I miss how to do this with the existing responder code? Are there
serious reasons why it should not be done anyway?

Michael