You will find attached herewith (or direct object)
Please find attached herewith (or direct object)
Attached you will find herewith (or direct object)
are set expressions used in business (even before the internet!) to mean a document is an attachment to a letter. Nowadays, an attachment to an email or a letter, or a letter in an email.
You attach a document to a letter or email is a regular grammatical form in English where attach is followed by a direct object. Here, I suspect that originally this form derives from French: Veuillez trouver ci-joint, as it is not a usual form in English but it widely prevalent in English business letters/communications.
When I was in my early twenties living in France, I first saw this in French and did not know it in English at all. It was only some years later that I realized it was used in English in what seems to be a direct translation. That's why I assume it originated in France and is probably a very old turn of phrase.
Please note that the object comes after the verb in this expression unlike other ways of using the verb attach in English: he attached the letter [object] to the document and mailed it out.
In a formal business letter the abbreviation Att.: [name of document attached] is placed on the left side at the end of the document.
Attached is the past participle. It is used idiomatically in this expression rather than with the usual grammar associated with an action verb: to attach something to something.
/Please find attached or enclosed/ or its variants given above would not normally even be grammatical in English. Nevertheless, the phrase has been around a long time.