If you’ve tagged it straight to a nominal code (e.g. “Something else not on the list”), then you may certainly be better off tagging it to an invoice. Documents can only be attached to invoices (or estimates).
What you’ve suggested sounds about right, but there are a few ways to create the invoice itself - whichever way you do it comes down to the way you work and the way you find best for your situation.
But generally speaking:
- Create the invoice
- Untag the bank entry, and retag it as “Payment to supplier” (for a purchase invoice)
- Upload / Attach your file(s)
I’ve also included an article below which may help: