Resources transferred as text/plain

[paul.wilding]

Whenever I load a page on trac which uses other resources (javascript, css, images etc) such as plugins or a ticket I get an error message(s) saying that one or more of these resources have been interpreted correctly but transferred with the MIME type text/plain. This occasionally causes a page to render incorrectly in the browser (chrome). 

I'm struggling to find the root of this problem - I have added 'AddType' lines to the httpd.conf file of Apache for the appropriate filetypes and have specified the mime types in the add_script and add_stylesheet calls in the python files of plugins and I still get the same issue. Does anyone have any other ideas of what this could be?