We had a DotNetNuke client report today that they could not directly access .xml or .swf files that they could previously get to from their portal. So of course the first thing we did verify the files in question were actually on the file system, they were. We knew this particular client had moved hosts recently so we checked al the IIS configurations and the file mime types were registered and allowed to be served so that was not it. We were then made aware that the sites 3rd party URL rewriting had recently been updated. This peaked our interest an after a little digging we discovered that with this particular URL rewriter you had to explicitly ignore file extensions through regular expression matching exposed as a property on the friendly URL provider. After adding the ignores for said extensions everything was back to normal. So if you see anomalies in content being served and you have verified IIS is setup correctly. Then the next place to look on a DotNetNuke installation is typically the portals friendly URL provider.