Convert DNG image attachments to PNG on web and desktop

[kbdevs]

Would maintainers approve adding automatic DNG-to-PNG conversion for image attachments on web and desktop?

DNG camera photos currently become generic file attachments when their MIME type is missing, or are rejected as unsupported images. The requested behavior is to drag, paste, or select a .dng photo and have T3 Code convert it locally to PNG before uploading, so the provider receives a supported image.

Proposed scope:

• Extend the existing shared web/desktop attachment preparation path, which already converts HEIC/HEIF.
• Load LibRaw/WASM only for DNG files and decode in a worker. Keep PNG output when resizing to the attachment limit.
• Apply a 50 MiB source limit, a 64-megapixel sensor limit, a decode timeout, and worker cleanup.
• Leave native mobile conversion for separate work.

An implementation is available for review in #16659. It has 66 passing focused tests, passing web typecheck and targeted lint, and a production-bundled smoke test using a generated DNG that verifies PNG output, resizing, and malformed-input rejection. An interactive browser pass has not been performed.

The PR was opened before obtaining the prior approval required by the contribution guide. This discussion requests that approval; the existing implementation is a concrete proposal, not an assumption that the feature or dependency is accepted.

Is this direction and scope acceptable? In particular, is a lazily loaded LibRaw/WASM dependency appropriate, or would you prefer a different conversion approach?