I would suggest "enclosed"; attached may indicate that the document is affixed. (In e-mail applications, however, "attached" is often used).

My dictionaries and referene books all seem to prefer enclosed.

Only Longmans Dictionary of Contemprary English even admits 'attach' in this sense, as in 'please attach a photograph to your application'.

I'd play safe with a ministerial ruling, though I personally like 'attached' for something weightier than the letter it is 'attached to' (in content if not in volume).