The file you posted has a 98 byte header before the start of the JPEG image.

It is interesting the Photoshop and GraphicConverter will both read this file, but only after I add a .JPG extension. Even though ExifTool does not rely on the extension to read the file, it is still possible to recognize files such as this. I have added the ability to skip past an unknown header in JPEG and TIFF images, and the feature will appear in version 6.04 when it is released. I have made a 6.04 pre-release available for testing purposes if you want to try it out before the official release (which should be in a week or so).