public ActionResult GeneratePPT(MyParams deckParams)
  {
      string template = Server.MapPath("/PPTTemplate.pptx");
      var results = _model.GeneratePPT(template);
      return File(results.Content, "application/vnd.ms-powerpoint", results.FileName);
  }

using System;
using System.Web.Mvc;
using System.Net.Mime;

namespace CoverageWebApp.Controllers
{
    public class PttDownloaderController : Controller
    {
        public FileResult GeneratePPT(MyParams deckParams)
        {
            try
            {
                //do something with deckParams...
                string template = Server.MapPath("/PPTTemplate.pptx");
                var results = _model.GeneratePPT(template);// provide _model

                return File(results.Content, MediaTypeNames.Application.Octet, results.FileName);
            }
            catch (Exception)
            {
                return null;
            }
        }
    }
}