JSESSIONID on URL causes an issue for the SE P1 handset

A JsessionID on the end of a download URL to a piece of content seems to cause a problem for the Sony Ericsson P1 handset e.g. file.3gp;jsessionid=xxxxx. The handset doesn't seem to recognise the format of the file and gives an error "There is a problem opening the file. Try Again?". The installation message sent back to the handset though is a 900 Successful Installation. Oddly if DRM is applied to the content (which will then be a .dm or .dcf file), the JsessionID is not really a problem. Removing the jsessionid from the URL to the content fixes the problem described above.