.thm - Canon thumbnail images
A simple way to view these files is to drop them on File Juicer and let it extract the JEPG inside. The files are typically tiny - just thumbnils to show on the built in display on the camera. There is nothing more than the JEPG inside and you can also just rename them to JPG if you just want to look at one of them.
.thm - Sony Theme Creator Files
You can decompress these files by renaming them to .tar and double clicking them in Finder.
Sony Theme Creator has been available for macOS, and maybe you can still find it