Indeed that is a "tough one."
There are several ways I can think of doing this but first here are a list of questions,
1) Jeff already raises the question of how you know the batch is finished.
2) I'd add does your email go to the creator of the files? Seems a bit strange since the creator knows about the files.
3) what's the relationship between files uploaded and the email? What would happen if user a uploads 2 files, while user b uploads a few more.
4) Or is the requirement to simply know the last 5 files uploaded. Or is that last 5 files per site?