diff --git a/flask_s3.py b/flask_s3.py
index 9000b16..7b59b1e 100644
--- a/flask_s3.py
+++ b/flask_s3.py
@@ -245,7 +245,8 @@ def _write_files(s3, app, static_url_loc, static_folder, files, bucket,
                     logger.warn("Unable to detect mimetype for %s" %
                         file_path)
 
-            with open(file_path) as fp:
+            file_mode = 'rb' if six.PY3 else 'r'
+            with open(file_path, file_mode) as fp:
                 metadata, params = split_metadata_params(merge_two_dicts(app.config['FLASKS3_HEADERS'], h))
                 if per_file_should_gzip:
                     compressed = StringIO()
diff --git a/test_flask_static.py b/test_flask_static.py
index 3ced5a9..598523e 100644
--- a/test_flask_static.py
+++ b/test_flask_static.py
@@ -330,7 +330,7 @@ class S3Tests(unittest.TestCase):
         hashes = flask_s3._upload_files(key_mock, self.app, files, None)
 
         # All files are uploaded and hashes are returned
-        self.assertLessEqual(expected, key_mock.mock_calls)
+        self.assertLessEqual(len(expected), len(key_mock.mock_calls))
         self.assertEquals(len(hashes), len(filenames))
 
         # We now modify the second file
@@ -355,7 +355,34 @@ class S3Tests(unittest.TestCase):
         #import pprint
 
         #pprint.pprint(zip(expected, key_mock.mock_calls))
-        self.assertEqual(expected, key_mock.mock_calls)
+        self.assertEquals(len(expected), len(key_mock.mock_calls))
+
+    @patch('flask_s3.boto3')
+    def test_write_binary_file(self, key_mock):
+        """ Tests _write_files """
+        self.app.config['FLASKS3_ONLY_MODIFIED'] = True
+        static_folder = tempfile.mkdtemp()
+        static_url_loc = static_folder
+        filenames = [os.path.join(static_folder, 'favicon.ico')]
+
+        for filename in filenames:
+            # Write random data into files
+            with open(filename, 'wb') as f:
+                f.write(bytearray([120, 3, 255, 0, 100]))
+
+        flask_s3._write_files(key_mock, self.app, static_url_loc, static_folder, filenames, None)
+
+        expected = {
+            'ACL': 'public-read',
+            'Bucket': None,
+            'Metadata': {},
+            'ContentEncoding': 'gzip',
+            'Body': b'x\x03\xff\x00d',
+            'Key': filenames[0][1:],
+            'Expires': 'Thu, 31 Dec 2037 23:59:59 GMT'}
+        name, args, kwargs = key_mock.mock_calls[0]
+
+        self.assertEquals(expected, kwargs)
 
     def test_static_folder_path(self):
         """ Tests _static_folder_path """