Use \xFF in bin file test

IPython/html/tests/test_files.py

@@ -76,7 +76,7 b' class FilesTest(NotebookTestBase):'
             write(nb, f, format='ipynb')
 
         with io.open(pjoin(nbdir, 'test.bin'), 'wb') as f:
-            f.write(b"\x5F"*3) # \x5F = _
+            f.write(b'\xFF' + os.urandom(5))
             f.close()
 
         with io.open(pjoin(nbdir, 'test.txt'), 'w') as f:
@@ -91,7 +91,7 b' class FilesTest(NotebookTestBase):'
         r = requests.get(url_path_join(base, 'files', 'test.bin'))
         self.assertEqual(r.status_code, 200)
         self.assertEqual(r.headers['content-type'], 'application/octet-stream')
-        self.assertEqual(r.text, '___')
+        self.assertTrue(r.content.startswith(b'\xFF'))
 
         r = requests.get(url_path_join(base, 'files', 'test.txt'))
         self.assertEqual(r.status_code, 200)