in addition to the previous reply...in fnd_attached_documents, the column_name will be your object area (i.e. AP_INVOICES) and pk1_value will be its ID...so invoice_id since it looks like you are working with invoices, assuming you are attaching at the header level.
Yes it is an invoice.
select * from fnd_attached_documents where pk1_value='xxx'
no rows selected
And how to find the tables related to an open form ?
One thing you can always do that works most of the time is open the form in question and then from the top menu go to Help>Record History.
This will tell you the table name (usually a view) that the form is using. I don't have access to the invoice form, but for POs the attachment form shows table name: FND_ATTACHED_DOCS_FORM_VL...guessing it would be the same or very similar. If you look at the source for that view in the database it will give you what tables the form is using.