First image is render setting. And second image is output file. That file should be png file but, it is not.
Windows (and other operating systems) doesn't know what to do with files that have no file extension.
Just rename the file and the extension that corresponds with the file format (in this case .png,), so that the OS understands it is dealing with an image (or video) file and uses the appropriate program to open such file.
In the future enable the File Extensions box on the output so that you get the name with a .png (or .exr or .jpg or mp4 or whatever) extension.