Make the MIME-type of downloads available

An application currently have to rely on suggested filename only to
guess the type of a download. This is often insufficient when the suffix
is misleading or missing.

This patch adds the mimetype to the reported metadata and also adds
a mimetype appropriate suffix to suggested filenames.

Task-number: QTBUG-48206
Change-Id: I4c70f076d6eb5ae820fd6b7f568515eeb7c18df5
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>