To re-process any record when its input value changes, go to:
Step 2: Extension Settings.
Enable "Re-run extension on rows if input changed".
If you're using the URL to attachment extension, you'll notice that -by default- it's not downloading the updated file from the URL. If you're trying to re-upload the attachments when the source URL changes, you'd need to enable this option from the settings.
Otherwise, the extension will skip any record that's filled already. It will only process records with an empty attachment field.