I appreciate you posting here in the Community, Tami. QuickBooks Online attaching files to a paycheck is currently unavailable. As a workaround, you'll want to consider attaching it to the employee's profile. This way, they'll be able to view the mileage log if you've invited them to QB Workforce.
Here's how:
Go to the Payroll menu.
Select the Employees tab, then open the appropriate employee.