Correct spelling, explanation: although for some ESL speakers, sound tch seems needed in /tʃ/ sound (as in catch or pitch), this is not always the case, and more importantly – it is not the rule. Attach comes from Middle English atachen and ths spelling remained, therefore the correct form is attach, not attatch.

Definition of attach:
verb, to add something, to include
Please attach your resume to your e-mail.
I will attach a key ring to my keys so I can find them easier.

Incorrect spelling, explanation: the correct form, attach, has roots in Middle English atachen. /tʃ / sound doesn’t have to be spelled with t; considering this fact and the old spelling attatch is not correct. The correct form is attach.