httpFileCollection uploads=HttpContext.Current.Request.Files;

//Store
Session["UploadedFiles"] = uploads;

//Retrieve
if (Session["UploadedFiles"] != null)
{
  //try-catch blocks omitted for brevity. Please implement yourself.
  HttpFileCollection myUploads = (HttpFileCollection)Session["UploadedFiles"];

  // Do something with the HttpFileCollection (such as Save).

  // Remove the object from Session after you have retrieved it.
  Session.Remove("UploadedFiles");
}