Added support for dng files

[bitblaster]

I thought it would be great to be able to sync also raw files, which many phones support these days.

I checked that the current exif data retrieval works with both Adobe_DNG_Converter-generated files and with native dng photos of my Oneplus3, so I simply added the dng extension to the image supported. I think this probably will also work with other raw formats, but I have no other phones to try.