byte[] getContent(String filename) {
        try {
            return Files.readAllBytes(Paths.get(filename));
        } catch (IOException e) {
            throw new RuntimeException(format("Can't read content of %s", filename), e);
        }
    }
@Test
    public void readNonExistingContent() {
        RuntimeException ex = Assertions.assertThrows(
                RuntimeException.class,
                () -> render.getContent("__file_that_may_not_exists_else_it_will_fail")
        );

        MatcherAssert.assertThat(ex, allOf(
                hasMessage(containsString("Can't read content of __file_that_may_not_exists_else_it_will_fail")),
                hasCause(instanceOf(IOException.class))
        ));
    }