Use BytesIO for bytes.

This fixes a TypeError on Python 3 that ocurred when passing
bytes as the values for files.

tests/test_requests.py

@@ -7,7 +7,6 @@
 import sys
 import os
 sys.path.insert(0, os.path.abspath('..'))
-
 import json
 import os
 import unittest
@@ -1030,7 +1029,9 @@ class RequestsTestSuite(TestSetup, TestBaseMixin, unittest.TestCase):
 
     def test_post_fields_with_multiple_values_and_files_as_tuples(self):
         """Test that it is possible to POST multiple data and file fields
-        with the same name."""
+        with the same name.
+        https://github.com/kennethreitz/requests/pull/746
+        """
 
         data = [
             ('__field__', '__value__'),
@@ -1061,6 +1062,10 @@ class RequestsTestSuite(TestSetup, TestBaseMixin, unittest.TestCase):
         self.assertEqual(body.count('__value__'), 4)
         self.assertEqual(body.count(file_field), 2)
 
+    def test_bytes_files(self):
+        """Test that `bytes` can be used as the values of `files`."""
+        post(httpbin('post'), files={'test': b'test'})
+
 
 if __name__ == '__main__':
     unittest.main()