Reason:

Some Firewalls do not allow certain mime types to pass.

Solution:

If your Firewall is blocking requests due to their content type, please add the mime type “application/x-shockwave-flash” for the filetypes .swf, .plg, .nlm, .skn and “application/xml” for .fsi and .xml to your webserver configuration.

e.g. for Apache mime.types file:

application/x-shockwave-flash swf plg nlm skn
application/xml xml xsl fsi