raw.json: add image/tiff to supported mimetypes
QImageIOPlugin need to specify the mimetype they support. Ref: https://doc.qt.io/qt-6/qimageioplugin.html
It was missing image/tiff, (that's the mimetype of the dng file in the bug https://bugs.kde.org/attachment.cgi?id=163279).
It might not be sufficient to fix the bug, but it sure does look like something missing. Depending on system configuration the mimetype might not be the same detected.
Untested as of now, testing is welcome.