clean up file handle

mkt/developers/tests/test_utils_.py

@@ -1,3 +1,5 @@
+from django.core.files.storage import default_storage as storage
+
 import amo.tests
 from mkt.developers.utils import check_upload
 
@@ -10,6 +12,6 @@ class TestCheckUpload(amo.tests.TestCase, amo.tests.AMOPaths):
             check_upload([], 'graphic', 'image/jpg')
 
     def test_valid(self):
-        errors, hash = check_upload(open(self.preview_image(), 'r'),
-                                    'preview', 'image/png')
-        assert not errors
+        with storage.open(self.preview_image()) as f:
+            errors, hash = check_upload(f, 'preview', 'image/png')
+            assert not errors