LocalPreview typedef
One preview video and the poster frame it was told to use.
frameTimeCode is null when the tree named none, and the null is carried
rather than resolved to defaultPreviewFrameTimeCode here: the uploader
prints "defaulting to" for one and the chosen value for the other, and a
model that filled it in would make those two cases indistinguishable at the
point where the difference is worth saying out loud.
Implementation
typedef LocalPreview = ({File file, String? frameTimeCode});