IMHO, script would probably be a feasible solution if it was a File(NFS) adapter. But since this is an SFTP adapter, the files are on the SFTP server, so the script would need to written to execute connection to the server to perform SFTP commands. It just unnecessarily complicates the design.
In general, I would always avoid usage of scripts wherever possible. Execution of scripts are independent of the channel and do not provide visibility in terms of monitoring as any errors are not logged back to the adapter framework, thus possibly introducing a gap in terms of end to end monitoring.