diff --git a/flask/helpers.py b/flask/helpers.py
index 56a91dd8..4034112e 100644
--- a/flask/helpers.py
+++ b/flask/helpers.py
@@ -513,7 +513,8 @@ def send_file(filename_or_fp, mimetype=None, as_attachment=False,
 
     if mimetype is None:
         if attachment_filename is not None:
-            mimetype = mimetypes.guess_type(attachment_filename)[0]
+            mimetype = mimetypes.guess_type(attachment_filename)[0] \
+                or 'application/octet-stream'
 
         if mimetype is None:
             if attachment_filename is not None:
diff --git a/tests/test_helpers.py b/tests/test_helpers.py
index fc85625e..e1469007 100644
--- a/tests/test_helpers.py
+++ b/tests/test_helpers.py
@@ -402,9 +402,7 @@ class TestSendfile(object):
             assert 'no filename is available' in str(excinfo)
 
         with app.test_request_context():
-            with pytest.raises(ValueError) as excinfo:
-                flask.send_file(StringIO("LOL"), attachment_filename='filename')
-            assert "Unable to infer MIME-type from filename 'filename'" in str(excinfo)
+            flask.send_file(StringIO("LOL"), attachment_filename='filename')
 
     def test_send_file_object(self):
         app = flask.Flask(__name__)