FileUploader: files not fully synchronized with underlying input

[brunnerh]

In the "edit" state, files can be removed, but the invisible file input will still contain them, so with a regular form post, they are still submitted. Programmatically changing the files property will also have no effect on the input. FileUploaderDropContainer should have the same issue.

REPL demo

A note on a potential fix approach:
The input.files property can be set, but FileList objects cannot easily be constructed (at least last time I checked). A common way around this is to use a DataTransfer.

const data = new DataTransfer(); 
data.items.add(file);
input.files = data.files;

Related/overlap with:

• FileUploader: Files are replaced rather than added #1774