client: don't cache one type in QWaylandMimeData
"application/vnd.portal.filetransfer", which is only valid for one time use.
Fixes: QTBUG-107858 Pick-to: 6.7 6.6 6.5 6.2 5.15 Change-Id: I094de22ea0bb9b3577572e6c57ebe42cdc8c7b41 Reviewed-by: David Edmundson davidedmundson@kde.org (cherry picked from commit 19758eb4)