DNG - Digital Negative File Format
This is an open format created by Adobe. Adobe about DNG
If you have macOS 10.4 File Juicer will use Apples RAW importer to convert the high resolution data to JPEG or TIFF depending on the checkboxes you set in the preferences.
To convert and work with the raw data and convert it to other formats you can also use iPhoto, Preview or dedicated professional applications like: