Jaishree,
The second one is correct because it contains the auxiliary verb ("is") and punctuation (the period or full-stop at the end). Without the verb, in particular, the first line is not actually a sentence. For clarity, you can always rearrange the wording into the basic subject + verb + object structure. For example, instead of "Attached is the completed file for your review" (no period, as my sentence isn't finished), you can write "The completed file is attached for your review". "The completed file" (subject) + "is attached" (verb) + "for your review" (object, or the point of attaching the completed file to the e-mail).
Also remember that the completed file requires someone to attach it to the e-mail, as it can't do it itself! :) When you leave out who is doing the attaching, you must always use the auxiliary ("to be") and the past simple of the main verb ("attached"). In this instance, you would use the present form of the auxiliary ("is") as the completed file is still attached to the e-mail when the other person receives it (and if it isn't, it's likely the fault of the wonders of technology and not of the wonders of grammar!).
I'm a bit rusty at the explanations, so I hope this is clear.
Edel