FINSYNC does not allow you to attach receipts to payments directly. You must create a bill with an attached receipt and then match the payment to the bill.
Follow these steps:
1) Go to "Expenses" in the top nav.
2) You will default to the "Payables" section of Expenses. On the far right, choose "New" and then "Bill." Note: you can also start this workflow from the inbox found under the "New Bills" indicator.
3) Fill out the required fields on the bill: Vendor, General Ledger Account, and Amount.
4) Click "Attach File" under the "Total Due" section.
5) Find the receipt on your system and select it. You'll see the green success indicator and the image link on the bill.
6) Save the bill.
7) You now have a bill in an unpaid state (temporarily).
8) Go to the bank account or credit card account from which the payment funds were withdrawn (by debit card, credit card, check, etc.).
9) Find the payment transaction and click the dropdown on its row on the far right.
10) Choose "Apply to Payable."
11) Find the bill in the list of unpaid bills and check the box on its row.
12) The green indicator should say "$0.00 Left." Click "Match."
You now have a payment matched to a bill with an attached receipt!