The problem still exists after installing 4.0.18. I'm using W10 64 bit. See attached pictures from Windows Explorer.
It seems to me that the file-suffix is amended through PaintdotNet.
After removing version 4.0.17 (??) all *.jpg files appeared oké in Windows Explorer.
Knipsel.jpg presents detailed mode of Explorer and Knipse_1 are large pictograms.
HTH