We get a notification when a diagram is created because the gliffy macro is inserted into the page causing a page updated event. When an attachment is saved there is a AttachmentUpdatedEvent created in the AttachmentManager, but the NotificationListener only listens to add and remove attachment events, hence no notifications.
Looking at the code that does the update of attachments from the web interface, they trigger their own AttachmentCreatedEvent (even if its an update) so thats what we need to add to the gliffy code.
Unfortunately at some point Atlassian have moved the EventManager class into its own package, therefore this will need to be implemented as a versioned method because of the different package names 
Shannon, can you please investigate this?