Concerning the article, I would include it. In fact, I would write:
Please find the daily report in the attachment.
You can find the daily report in the attached file.
When I think of the word in used without an article, I think of phrases such as in love, in jest, in concert. In such cases, the word in essentially means "in a state of."
However, when using the word in to mean "inside of something," I would normally expect to see an article: in the house, in a box, in the file, and in the attachment.
Concerning which article to use, I would favor in over at. However, we are dealing with bits stored in cyberspace, where locations are a bit muddled, so my ears aren't all that much bothered by using at, either.
Where can I find the daily report? I can find it at the attachment.
As I said, I much prefer in, but I don't find at to be utterly unacceptable.