The list of documents available is set by the document type that you are attempting to attach to. If you want to amend this list, you can do the following:
Open the ADM Client
Navigate to System Control, Integration, Integration Links
Select the Integration link of your database and press Edit Integration Link
Select Searching, Scanning and Filing, find the Document type you wish to change and press the Edit button
Select the Filing Defaults tab, In the Document Types section, tick the document type you wish to add and press OK