### Description

The example reads a vtkUnstructuredGrid with vtkXMLUnstructuredGridReader if the extension is .vtu or the legacy vtkUnstructuredGridReader if the extension is .vtk. If a file is not present, the example creates a vtkUnstructuredGrid by passing a vtkSphere through vtkAppendFilter.

For a description of the VTK File Formats see [VTKFileFormats](/VTKFileFormats).
