Sorry, but is this issue supposed to be fixed ? I don't understand the "no need to be fixed".
- open XnviewMP 1.0
- within the browser select a folder with different format of files or images.
- type "jpg" in the Quick search field -> only jpeg images displayed
- open one of these image in viewer
- use arrows to scroll to the next images.
Sooner or later viewer will display a no jpeg image from the folder

Thanks for listening !