Makes sense, but at least this would generally be out of a normal users usage case (multi-file documents), and so the power user could probably just open flatseal.
I would not be so sure. Firefox has a “save web page as…” option which saves the html page and all other files needed into a sub folder.
Without better handling of reading and writing files the sandbox will break that builtin function. another way of working around this. would be to change firefox to save the web page into one file. Maybe something a .html+zip file that firefox would know how to open. However that would lock other browsers out without manualy unziping it first.
Getting sandboxing right with powerful programs is very hard and I feel the tooling is still not here yet.