It is a extension of Microsoft Office Temporary file. It may be viewed by Microsoft Office Word.
While Microsoft Office tools are processing there is a hidden file that is used as backup. These files take the same name with the file that you are working on. But with the addition of ~$ni symbol to the file extension.
If the Office Word software crashes it may leave a hidden file. Before running Microsoft Word you should change the name of hidden file and delete the ~$ letters. Then you make a double click and try to open. The chance of saving the file is small but you can try and save your work.
You can use Open Office software to open the files with $ni.
What to use ~$ni
Microsoft Office Word
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