diff --git a/locust/test/testcases.py b/locust/test/testcases.py
index adfbffcdc6..83ea465e93 100644
--- a/locust/test/testcases.py
+++ b/locust/test/testcases.py
@@ -15,7 +15,7 @@
 
 def safe_repr(obj, short=False):
     """
-    Function from python 2.7's unittest.util. Used in methods that is copied 
+    Function from python 2.7's unittest.util. Used in methods that is copied
     from 2.7's unittest.TestCase to work in python 2.6.
     """
     _MAX_LENGTH = 80
@@ -90,7 +90,8 @@ def basic_auth():
 
 @app.route("/no_content_length")
 def no_content_length():
-    r = send_file(BytesIO("This response does not have content-length in the header".encode('utf-8')), add_etags=False)
+    r = send_file(BytesIO("This response does not have content-length in the header".encode('utf-8')), mimetype="application/octet-stream",
+                    add_etags=False)
     return r
 
 @app.errorhandler(404)
@@ -123,11 +124,11 @@ def setUp(self):
             event = getattr(events, name)
             if isinstance(event, events.EventHook):
                 self._event_handlers[event] = copy(event._handlers)
-                      
+
     def tearDown(self):
         for event, handlers in six.iteritems(self._event_handlers):
             event._handlers = handlers
-    
+
     def assertIn(self, member, container, msg=None):
         """
         Just like self.assertTrue(a in b), but with a nicer default message.
@@ -137,7 +138,7 @@ def assertIn(self, member, container, msg=None):
             standardMsg = '%s not found in %s' % (safe_repr(member),
                                                   safe_repr(container))
             self.fail(self._formatMessage(msg, standardMsg))
-    
+
     def assertLess(self, a, b, msg=None):
         """Just like self.assertTrue(a < b), but with a nicer default message."""
         if not a < b:
@@ -162,7 +163,7 @@ def assertGreaterEqual(self, a, b, msg=None):
             standardMsg = '%s not greater than or equal to %s' % (safe_repr(a), safe_repr(b))
             self.fail(self._formatMessage(msg, standardMsg))
 
-            
+
 class WebserverTestCase(LocustTestCase):
     """
     Test case class that sets up an HTTP server which can be used within the tests