We cache the files retrieved by the public URLs for attachments automation for up to 5 days.
This means that if you open a URL, change the image/video/document in Airtable, and then open it again, you'll get the old attachment for up to 5 days. The new file will be returned afterwards.
You can 'refresh' the cached attachment by creating a new record in Airtable and uploading your attachment. This will regenerate a new URL that works instantly.