Is it possible to send an uploaded file (attachment) to an Enterprise Server via an Outbound Message?
Even if you could add an attachment to the message you wouldn't be able to use anything in the standard UI to be able to find the appropriate file. Your best bet here would be to use Apex to make the callout to the remote service, activating it either via a trigger on attachment if that's what you're using, or possibly from a VisualForce page where you use an
<apex:inputFile> element to facilitate the upload part.
I can't really be more specific without more details but hopefully this will get you headed in the right direction.