Files must be uploaded to temp storage, we cache the result for 1 hour. This means that if you open a URL, change the image/video in Airtable, and then open it again, you'll get the old attachment for an hour. The new file will be returned afterward.
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.