This is a configuration problem with the mime type of your web server - it doesn't have json, probably. Try renaming the file extension to .txt or .html and it should work.
You can also add a mime extension to the server. For IIS express, this is web.config. For instance:
<staticContent>
<remove fileExtension=".json" />
<mimeMap fileExtension=".json" mimeType="application/json" />
</staticContent>
source
share