How do I compress a Json result from ASP.NET MVC with IIS 7.5

Question

I'm having difficulty making IIS 7 correctly compress a Json result from ASP.NET MVC. I've enabled static and dynamic compression in IIS. I can verify with Fiddler that normal text/html and similar records are compressed. Viewing the request, the accept-encoding gzip header is present. The response has the mimetype "application/json", but is not compressed.

I've identified that the issue appears to relate to the MimeType. When I include mimeType="*/*", I can see that the response is correctly gzipped. How can I get IIS to compress WITHOUT using a wildcard mimeType? I assume that this issue has something to do with the way that ASP.NET MVC generates content type headers.

The CPU usage is well below the dynamic throttling threshold. When I examine the trace logs from IIS, I can see that it fails to compress due to not finding a matching mime type.

<httpCompression directory="%SystemDrive%\inetpub\temp\IIS Temporary Compressed Files" noCompressionForProxies="false">
    <scheme name="gzip" dll="%Windir%\system32\inetsrv\gzip.dll" />
    <dynamicTypes>
        <add mimeType="text/*" enabled="true" />
        <add mimeType="message/*" enabled="true" />
        <add mimeType="application/x-javascript" enabled="true" />
        <add mimeType="application/json" enabled="true" />
    </dynamicTypes>
    <staticTypes>
        <add mimeType="text/*" enabled="true" />
        <add mimeType="message/*" enabled="true" />
        <add mimeType="application/x-javascript" enabled="true" />
        <add mimeType="application/atom+xml" enabled="true" />
        <add mimeType="application/xaml+xml" enabled="true" />
        <add mimeType="application/json" enabled="true" />
    </staticTypes>
</httpCompression>

Answer 1

Make sure your %WinDir%\System32\inetsrv\config\applicationHost.config contains these:

<system.webServer>
    <urlCompression doDynamicCompression="true" />
    <httpCompression>
      <dynamicTypes>
        <add mimeType="application/json" enabled="true" />
        <add mimeType="application/json; charset=utf-8" enabled="true" />       
      </dynamicTypes>
    </httpCompression>
</system.webServer>

From the link of @AtanasKorchev.

As @simon_weaver said in the comments, you might be editing the wrong file with a 32 bit editor on a 64 bit Windows, use notepad.exe to make sure this file is indeed modified.