The APIs:
QFileDialog::saveFileContent
QFileDialog::getOpenFileContent

are not well suited for future improvements as they expect a QByteArray instead of a QIODevice. This makes it hard to support transfering large files in the future. I'm not sure whether it would currently be possible to do so, but IMHO handling this as QIODevice would make this future proof, even if the WebAssembly APIs only allow byte arrays here.

Imagine you wan to upload or download a video. This might not fit into memory and will probably lead to a crash with this API.