ASP.NET - Passing JSON from jQuery to ASHX

I'm trying to pass JSON from jQuery to a .ASHX file. Example of the jQuery below:

$.ajax({
      type: "POST",
      url: "/test.ashx",
      data: "{'file':'dave', 'type':'ward'}",
      contentType: "application/json; charset=utf-8",
      dataType: "json",      
    });

How do I retrieve the JSON data in my .ASHX file? I have the method:

public void ProcessRequest(HttpContext context)

but I can't find the JSON values in the request.