You're correct that it is there. But when you sync attachments, the problem is it returns in a format like this... (null, '80', null, false, null, null) in a field called AttachableRef.
I attached docs to our last example (credit card expenses) as well as an invoice and check, and the number 80 in that value is the related transaction ID. In truth, I haven't tested this enough to know for certain what the other elements of that response in AttachableRef represent.
You can isolate that number (which is the linked transaction) by creating a formula text field and create the formula as follows...
NotLeft(NotRight([AttachableRef],"'"),"'")
The next step is to build a relationship between your documents table, and (in the case of our credit cards) the expenses table, where an expense can have many Docs. What you want to do though is to change the default reference field Quick Base selects to whatever you named your new TXN ID formula field.
***NOTE*** Since that transaction number is NOT JUST FOR EXPENSES, you may need to get creative with this by building the same type of relationship again for each "Type" of transaction you're syncing, ie, invoices, checks, payments, bills etc., but this should help you get started....