Date of Attachment Files, e.g. *.pdf

The date when stored ist always the actual date, although the files are old. That does not lead to truth and provability…

Yes, it will be the date and time you save the file on your device.

It is not the same as moving or copying files in Explorer. Attachments are not actual files, but encoded text, that will need to be recreated into a file when you save the attachment. So in essence it is a new file.