Fixed in r4395
For download of modified cgi/client.py, see
http://svn.roundup-tracker.org/viewvc/roundup/roundup/trunk/roundup/cgi/client.py?view=log

Turns out that the url .../msg1/ requests download of the file in the
content property of a message. For messages the type property (which
should contain the mime time) is empty. That means we get the python
value None in one of the header fields which led to the traceback.

Fix: If the mime type is None we set it to text/plain. This only happens
for messages (files *do* have a mime type set).