When you copy details from one period to another, and attachments were added in the original period, then these attachments were copied as well to the new period.
In most cases, the attachments of the old period are irrelevant in the new period.
So, by using the roll forward function (more info on that here), you can now make it so that attachments will not be copied at all, by using this:
{% input custom.some.file as:file %}
{% rollforward nil custom.some.file as:file %}
When copying details, this will make sure no attachment is copied.