Using .JSON URL to return a JSON response

Hi Guys,

I'm looking to create a web application where a client can make a request via URL eg. http://www.example.com.au/atdis/1.0/applications.json?postcode=2089 and this should produce a .json file with all application in a specific postcode.      
Currently, I'm using asp.net (C#) that accepts a URL such as http://www.example.com.au/atdis/1.0/applications.aspx?postcode=2089 to search, generate the data and create the .json response:
 Response.ContentType = "application/json; charset=utf-8";  Response.AddHeader("Content-Disposition", "attachment;filename=\"application.json\"");
But the problem is that my URL is with .aspx and my client is requiring to send a URL with .json for their response ...