How to allow download of .json file with ASP.NET

When adding support for mimetype (as suggested by @ProVega) then it is also best practice to remove the type before adding it - this is to prevent unexpected errors when deploying to servers where support for the type already exists, for example:

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

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.

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

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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>

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.