Here are some examples:
* Attached to the email, you will find the document.
* Attached to this letter is a copy of my resume.
* The picture is attached to the wall.
However, other prepositions can sometimes be used, depending on the context:
* Attached with (meaning connected or fastened with something): The two pieces were attached with glue.
* Attached by (meaning connected by a specific person or thing): The file is attached by the sender.
The best preposition to use will depend on the specific meaning you want to convey.