XnView Software

Thanks for your time and solution.

The screenshots of the settings and status are shown below.

The filename is "{Filename}_result", and the output folder is "Source folder".

Software: XnView MP 0.94.1 Zip Win 64-bit
OS: Windows 10 LTSC - 64-bit

To reproduce:
1. Select a png image file.
2. Click "Tools - Batch convert".
3. Choose the "Output" subfield. Set "When output files already exist" as "Skip" and check “Use CPU Cores”. Choose the "format" as "JPG - JPEG / JFIF ...

Yes.

When I converted a png image to a jpg image, the extension name of the resulting image remained ".png", but not ".jpg".