A thread safe variant to get a custom file being set on the self.
Free the returned Gio.File, if not None, with GObject.Object.unref(),
when no longer needed.
Returns whether the backend should prefer to open the file
in writable mode. The default is True. The file can be still
opened for read-only, for example when the access to the file
is read-only.
Set whether the custom file should be opened in writable mode.
The default is True. The file can be still opened for read-only,
for example when the access to the file is read-only.