Not sure what your question is ;-). Yes, an alert could work, but did you try at all (and it doesn't work and you need help with that) or do you want someone to write the alert for you?
In this case you would add the alert on_insert of fnd_attached_documents I think (base of view from FND_ATTACHED_DOCS_FORM_VL) so you can trigger it on a specific entity_name (sales orders) or types.
Could you please show us how you created that event alert.
if possible post screen shots also, the we will check