The CTG file does not belong to any media file. It is an index file of photos in the camera. It can't be opened on the computer and needs to be read by a Canon camera.

Just insert the card into the Canon camera, which mainly provides some information that the camera needs to store/read the card. CTG files allow you to import files into your computer and take photos from your digital camera for printing and editing. Do not delete or attempt to open the CTG file. These files can only work properly in your camera memory card. When you delete the photos in the memory card, the CTG file will be deleted and completed automatically. Therefore, CTG files do not belong to any media files. It belongs to the index file of photos in the camera. It is recommended not to delete.