> What about undo that expires in 1 minutes? Gmail website does that for sent emails (for 5 seconds I guess).

Same question as parent: "And do what when tmp gets full and services & applications start crashing?"

You could do what is more or less an industry standard when it comes to this and warn the user that there's not enough room to allow an undo when deleting said file(s) and prompt if they want to delete it permanently instead.

It's not rocket surgery.