One of the most irritating server configuration issues I’ve run across recently emerged when adding global MIME type mappings to Microsoft Internet Information Services 7 — part of Windows Server 2008 R2.
Basically, if you have a MIME type mapping in a domain or path, and later add a mapping for the same file extension at a higher level in the configuration hierarchy, any subsequent requests to that domain or path will start returning HTTP 500 server errors.
You will not see any indication of conflicts, when you change the higher level MIME type mappings, and you typically only discover the error when a user complains that a specific page or site is down.
When you check your logs, you’ll see an error similar to the following:
\\?\C:\Websites\xxx\www\web.config ( 58) :Cannot add duplicate collection entry of type 'mimeMap' with unique key attribute 'fileExtension' set to '.woff'
Furthermore, if you try and view the MIME types in the path or domain that is faulting within IIS Manager, you will receive the same error and will not be able to either view or address the problem (e.g. by removing the MIME type at that level, which would be the logical way to address the problem). The only way to address the problem in the UI view is to remove the global MIME mapping that is conflicting — or manually edit the web.config file at the lower level.
Not very nice — especially on shared hosts where you may not control the global settings!