NAS542 encrypted with ransomware "checkmate" - what to do?

Hi, I today noticed that all my files on my NAS542 was not accessible and have an extension in the file type, eg. "picture.jpg.checkmate". All files encrypted by a ransomware ".checkmate". Anyone who knows what to do? Luckily I do have an external backup. However when I tried to open the files there are just folders in ".DAR"-format. But how do I open that file format so that the files can be accessed and read/viewed??