When adding a manual trip, you can only enter the date, total mileage, and trip purpose. Currently, the ability to attach receipts or any file in the mileage page is not available. What you can do is to attach the receipt on the Transactions page and accurately tagged it under car/vehicle expense or travel expense. Let me guide you on how to do it.
Go to the Transactions menu.
From the Transactions page, search for the transactions specific to fuel expenses.
Drag or drop the receipt.
You can refer to these articles for more detailed instructions about adding receipts and transactions: