As it stands, there is no way to view the preview embedded in a raw file except by loading the raw file into an external app.
Many a folk has wanted to view, in Lightroom, the camera-generated preview, or preview as edited by camera manufacturer software, or even DNG preview as previously saved in Lightroom or Photoshop/ACR.
So, the 'Idea' is for some way to temporarily view the jpeg preview embedded in a proprietary raw or DNG file.
Embedded preview should be usable in before/after comparison, as well as side-by-side, as well as temporary loupe display.
Note: development resources to satisfy this request would be relatively small.
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