What is the Chinese equivalent of "Encl." used in formal letters? What is the Chinese equivalent of "Encl." used in formal, written correspondences to alert the recipient of the presence of enclosed documents?
随信附上 or 附件 

Here is a webpage of baidu(an chinese website) zhidao (pinyin, which means know) which explains Encl. in chinese and english.
http://zhidao.baidu.com/question/40359245.html
Hope it is helpful. :)