Select does support .DNG files, however, it currently relies on the file's embedded JPG preview to display your image. However, if that preview JPG is very small, you'll see the image as pixellated/blurry and our face assessments may not work very well.
If your camera shoots .DNG files
Some cameras will allow you to change the embedded JPG preview size. If so, set this to Full Size/High Quality.
If you can't change this setting and you still want to use Select BETA, you can import them into Lightroom first, then export them with the settings described below.
If you are exporting .DNG files from Lightroom
If you are exporting your RAWs from Lightroom as .DNG files, make sure the JPEG preview is set to 'Full Size' in your export settings.
Eventually, we will have a comprehensive solution for this issue – so hold tight if you are unable to get this working.