You can run an initial activity from file listener to get the file content, pass it on clipboard using a parameter and assign that parameter to 'pyAttachStream' property which is used to attach the file content to theobject.
You can use below code in map to key activity in File listener, to capture the byte stream of the file and put it in a parameter.
However this would depend on how you are handling the outbound emails, for example I would have gone for a class derived from Data- to keep all my outbox emails before sending them. With this I can easily do an Obj-Open on all the instances of my outbox data class and add the file as an attachment (parsed by file listener) and send accross. Later those outbox data instances can be deleted when sent successfully.