Optimization in QMimeDatabasePrivate::mimeTypeForFile does not work for Custom File Engines

https://code.qt.io/cgit/qt/qtbase.git/tree/src/corelib/mimetypes/qmimedatabase.cpp#n446 introduced an optimisation to call QT_STAT directly instead of using QFileInfo on Q_OS_UNIX.

This does not work for custom FileSystemEngines.

When the QT_STAT returns != 0, it will then skip the initial check for if it is a directory and may try to open the path as a file and get its contents (which will fail for directories).

In case the QT_STAT fails, a check using QFileInfo::isDir should still be done first to return directoryMimeType().