ASP.NET MVC / IIS 7.5: 500 Internal Server Error for static content only

Question

An ASP.NET MVC project running under IIS 7.5 (but problem may exist on earlier versions and non-MVC sites) which was previously working has started returning 500 Internal Server

Answer 1

I enabled Failed Request tracing and after searching through the (enormous) log, I discovered this:

 
  {00000000-0000-0000-3700-0080010000FC}
  CustomErrorModule
  536870912
  500
  Internal Server Error
  19
  2147942583
  
    \\?\C:\Websites\xxx\www\web.config ( 58) :Cannot add duplicate collection 
    entry of type 'mimeMap' with unique key attribute 
    'fileExtension' set to '.woff'
  
 

What has happened is, a new Mime type for serving web-fonts (.woff) had been explicity added to the site.

Later on, the new Mime type (.woff) had also been added to the global IIS mime types.

Instead of the website setting overriding the global setting, an exception was being thrown.