AssertionError on images mime type

If you click on the preview link on the file history page, you get the following error:

Internal Server Error

The server encountered an unexpected internal server error

(generated by waitress)

You can see this bug here: https://karl.soros.org/communities/its-a-test/files/scan-9.jpeg/history.html

Error monitor shows:

Exception when serving /communities/its-a-test/files/scan-9.jpeg/download_preview

Traceback (most recent call last):
  File "/srv/osfkarl/production/47/eggs/waitress-0.8.9-py2.6.egg/waitress/channel.py", line 337, in service
    task.service()
  File "/srv/osfkarl/production/47/eggs/waitress-0.8.9-py2.6.egg/waitress/task.py", line 173, in service
    self.execute()
  File "/srv/osfkarl/production/47/eggs/waitress-0.8.9-py2.6.egg/waitress/task.py", line 392, in execute
    app_iter = self.channel.server.application(env, start_response)
  File "/srv/osfkarl/production/47/eggs/perfmetrics-2.0-py2.6.egg/perfmetrics/__init__.py", line 294, in app
    return nextapp(environ, start_response)
  File "/srv/osfkarl/production/47/eggs/karl-3.126-py2.6.egg/karl/timeit.py", line 73, in __call__
    return resp(environ, start_response)
  File "/srv/osfkarl/production/47/eggs/WebOb-1.1.1-py2.6.egg/webob/response.py", line 939, in __call__
    start_response(self.status, headerlist)
  File "/srv/osfkarl/production/47/eggs/waitress-0.8.9-py2.6.egg/waitress/task.py", line 376, in start_response
    'Header value %r is not a string in %r' % (v, (k, v))
AssertionError: Header value u'image/jpeg' is not a string in ('Content-Type', u'image/jpeg')