You are right that 'Please find the attached file' seems more logical grammatically.
However, the other construction is an established formula that goes back a long time. Before the days of email attachments, we wrote letters saying 'Please find enclosed:...' , followed by what you were enclosing in the envelope. This formula works better if you have a number of items. 'Please find attached' is a kind of heading, followed by a list of attachments.