@Override
  protected InputStream readInputStreamParam(String key) {
    Part part = readPart(key);
    return (part == null) ? null : part.getInputStream();
  }
@Test
  public void returns_null_when_invalid_part() throws Exception {
    when(source.getContentType()).thenReturn("multipart/form-data");
    InputStream file = mock(InputStream.class);
    Part part = mock(Part.class);
    when(part.getSize()).thenReturn(0L);
    when(part.getInputStream()).thenReturn(file);
    doThrow(IllegalArgumentException.class).when(source).getPart("param1");

    assertThat(underTest.readInputStreamParam("param1")).isNull();
  }