I run in to jpg files that microsoft viewer shows are unsupported formats now and then in my personal reference storage folder.

For a single unsupported jpg file try this.

Right click on the offending file and some options should appear.

pick: openwith: microsoft paint

The file should be now viewable in ms paint.

Now pick: save

It should ask if you want to replace it. Pick: yes

The format is rewrote and should be now viewable in image viewer.

If it does not allow it to be replaced pick: save as, and rename it, then save it. The original can be deleted.

(For mass conversions of files or a slide show the program XnView works.)