Overrides the dashboards using json instances from the file.

def import_dashboards(self):
        """Overrides the dashboards using json instances from the file."""
        f = request.files.get('file')
        if request.method == 'POST' and f:
            dashboard_import_export.import_dashboards(db.session, f.stream)
            return redirect('/dashboard/list/')
        return self.render_template('superset/import_dashboards.html')