Q:

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

How can I enable the download of *.json files from an old ASP.NET site (IIS6 I am led to believe)?
I am getting a 404 page instead of the JSON file.
Do I need to create a web.config file? What goes in it?

A:

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.

A:

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.

A:

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>

