ApplicationLauncherJob: find mimetype before showing open with dialog
To support "last used application" feature in Plasma Integration, it's
required to pass a mimetype name in promptUserForApplication
.
This also matches the behavior in KIO::OpenUrlJob
.
Implemented in KF5 because it's unlikely to break things and can support Qt5 applications to use the "last used application" feature" after Plasma 6 is released.
Required by: plasma/plasma-integration!76 (merged)