if is not an archive, always copy

Summary:
for wallpapers, some items can be an archive, some aren't
this may be true for other types of resources as well, so
make this more general. if the downloaded file is not an archive,
just copy it, instead of silently fail

Test Plan: different types of assets still install correctly, all wallpapers now install

Reviewers: Plasma, leinir

Reviewed By: leinir

Subscribers: plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D4118