Cannot add duplicate collection entry of type ‘mimeMap’ with unique key attribute ‘fileExtension’ set to ‘.json’

This issue occurred today while I was trying to run an Ext JS application from IIS, because .json files are not accepted by IIS I tried to add this file extension to the application. But instead I got a 500 error when trying to browse the application.

After some digging and searching the MIME types section of the website and all other applications, I couldn’t find the conflicting extension.

I was able to solve the issue by editing the application’s web.config file, right before the mimeMap tag, add a remove tag like following:

<staticContent>

<remove fileExtension=“.json“/>

<mimeMap fileExtension=“.json“ mimeType=“application/json“ />

</staticContent>

I still have no idea why this works, but it works.