You can't, since the file can be deleted from under you after verification anyway. But you can treat it as an "S3FileReference" or something arbitrarily generic

You can't, since the file can be deleted from under you after verification anyway. But you can treat it as an "S3FileReference" or something arbitrarily generic like "RemoteFile" and have associated procedures/methods for verifying the properties you want at any point without confusing it with "just a string"