Hi Hetal,
You'll need to do Full Trust for this. Why not just add a file attachment control and make it required so that users attach before submit? Then, you can be domain trust, which is the best option.
There is no way to programmatically force an attachment. However, you write some code to do that. An alternative is to just popup a dialog on submit telling them they need to attach a file with an explanation of how to do this using the file attachment control.
Good luck!
Patrick Halstead
Project Manager at Qdabra