The Confluence release notes for 5.10 includes:
We now send just one email with the changes that have happened during the batch window (10 minutes). You'll still receive immediate notifications for the most important things, like mentions and new pages.
see Confluence 5.10 Release Notes for more details.
However, this impacts only pages edits and not attachments updates. Every change made to page attachments is being automatically sent in an email.
Extend the page edits notification algorithm to the attachment-related notifications as well