We're on beta 4 and I still can't open a RAW photo file on my iPad 12.9" 2018 model that my iPhone 7 Plus can open running IOS 12.3.1. I can see a RAW photo in iPad apps like Amazon Photos and Lightroom but not if the file is on the iPad itself or on an attached SSD drive. The iPad will view RAW files perfectly if it's coming into the Photos app via an SD card reader transfer but not from an attached drive or a cloud service like OneDrive.
When will iPad open a RAW image?
iPadOS beta 5 (July 29, 2019) has taken care of the problem. I can view any RAW or JPG file in either the Photos app or in the Files app. Perfect!!