Add DNG as a valid photo filetype

[ianrahman]

First time contributor checklist

• I have read the README and CONTRIBUTING documents
• x] I have signed the Contributor Licence Agreement

Contributor checklist

• My commits are rebased on the latest main branch
• My commits are in nice logical chunks
• [ x] My contribution is fully baked and is ready to be merged as is
• I have tested my contribution on these devices:

• iPhone 16 Pro Max, iOS 26 beta 9
• iPad Mini 2024, iPadOS 26 beta 9

---

Description

Adds support for DNG as a valid image format. Resolves #4298 and #5337

Tested on two physical devices with RAW images captured on the iPhone and on a different DNG-generating camera.

[schmeat]

Nice! Current open ticket for it is #5337

[ianrahman]

@max-signal I see you self assigned the open ticket for this issue. Just wanted to point out this exists now.

[ianrahman]

@sashaweiss-signal @max-signal is there anything else I can do to help this along?

[max-signal]

@ianrahman when I investigated this last time, there were few changes we needed to make related to file sizes to resolve issues sending RAW files. The first of those was 425bf68, and there will be a few more to follow. I’ll circle back to this PR once I’ve finished the others.