Comment 5 for bug 1626667

You’re missing the following package: qtdeclarative5-ubuntu-download-manager0.1

But that’s not going to solve the lack of apps that provide this kind of content.
Because the webapp container is running confined, it cannot just offer a file picker that would allow browsing the file system. Instead, it relies on applications that act as content providers, to pick content from. For example the gallery-app package can provide images. For all types of file, you want the ubuntu-filemanager-app, but it doesn’t appear to exist as a debian package, only as click.