If you want to manually add support to your site, you can just add the following to your web.config in the system.webServer section:

<staticContent>    <mimeMap fileExtension=".json" mimeType="application/json" /></staticContent>

This will add a "local" configuration under IIS. This does not work in IIS6, but does work in IIS7 and newer.

Add the JSON MIME type to IIS 6. Follow the directions at MSDN's Configure MIME Types (IIS 6.0).

• Extension: .json
• MIME type: application/json

Don't forget to restart IIS after the change.

UPDATE: There are easy ways to do this on IIS7 and newer. The op specifically asked for IIS6 help so I'm leaving this answer as-is. But this answer is still getting a lot of traffic even though IIS6 is very old now. Hopefully you're using something newer, so I wanted to mention that if you have a newer IIS7 or newer version see @ProVega's answer below for a simpler solution for those newer versions.

Solution is you need to add json file extension type in MIME Types

Method 1

Go to IIS, Select your application and Find MIME Types

Click on Add from Right panel

File Name Extension = .json

MIME Type = application/json

After adding .json file type in MIME Types, Restart IIS and try to access json file

Method 2

Go to web.config of that application and add this lines in it

 <system.webServer>   <staticContent>     <mimeMap fileExtension=".json" mimeType="application/json" />   </staticContent> </system.webServer>