中译英：附件是盖有公章的合同。

The attachemnt is the contract with the company's stamp.
我们图纸、合同上的公章都叫stamp

Enclosed is the contract with official seal.

enclosed 还可以换成 attached

还可以翻译为 The enclosement is或者 The attachement is

The attachemnt is the contract with the company's seal.

attachment is contract with cachet

the signed contract is attached.

attached contact with company seal
contact with company seal as attachment