Last modified at 11/2/2013 12:00 PM by Koen Zomers
It may happen that when you try to make a file with a certain extensions available for download through your IIS webserver, that it will result in a 403.3 File Not Found error while you're certain it's there. This is caused by the way the StaticFile handler in IIS has been set up. It requires a MIME mapping to be available before it allows the file to be downloaded. Because IIS doesn't want to expose information about this to the outside world, it will only show a vague 403.3 File Not Found error as an output and in its internal logs.


In order to add the file extension you want to make available for download, follow the steps below:

  1. Log on to your IIS server and open up Internet Information Services (IIS) Manager

  2. Expand the Connections pane at the left and navigate to the IIS Site you wish to allow the extension on

  3. At the right doubleclick on the MIME Types icon in the IISsection


  4. In the Actions panel at the right, click on Add


  5. In the Add MIME Type dialog box, enter the extension you wish to allow. Be sure to prefix it with a dot. As the MIME type, enter:



    Click OK to close the dialog window

  6. The new extension should now be listed under the MIME Types


    Now try downloading the file again from your IIS webserver. It should now work: