Mocking apache-commons file upload with Mockito - build request manually

I'm trying unit test a servlet processing a xml file with the following code below and could use some direction on building the request manually :

@Test
public void testFacadeLADServlet() throws Exception {

    HttpServletRequest request = mock(HttpServletRequest.class);
    HttpServletResponse response = mock(HttpServletResponse.class);

    File file = new File("src/test/resources/LAD_PREMIA_PLUS_2050.xml");
    FileInputStream in = new FileInputStream(file);
    ServletInputStream srvInputStream = new MockServletInputStream(in);

    when(request.getContentType()).thenReturn("multipart/form-data; boundary=BbC04y7F7VsZ5f6");
    when(request.getCharacterEncoding()).thenReturn("UTF-8");
    when(request.getContentLength()).thenReturn(13312);
    when(request.getInputStream()).thenReturn(srvInputStream);

    PrintWriter writer = new PrintWriter("C:\\TEMP\\somefile.txt");
    when(response.getWriter()).thenReturn(writer);

    servlet.init(null);
    servlet.doPost(request, response);
}

MockServletInputStream is a wrapper class :

class MockServletInputStream extends ServletInputStream {

    private final InputStream sourceStream;

    public MockServletInputStream(InputStream sourceStream){
        Assert.assertNotNull("L'inputStream ne peut être null", sourceStream);
        this.sourceStream = sourceStream;
    }

    @Override
    public int read() throws IOException {
        return this.sourceStream.read();
    }

    @Override
    public void close() throws IOException {
        super.close();
        this.sourceStream.close();
    }
}

In the servlet :

ServletFileUpload fileUpload = new ServletFileUpload(defaultFileItem);
        fileUpload.setSizeMax(MAXSIZE);
        List<?> list = fileUpload.parseRequest(request);

The fileUpload.parseRequest(request) always returns an empty list.