I was trying to enable attachments to General journal lines yesterday and ran into a real challenge finding the correct table to add to the Active Document Tables, until I realized you have to search the 2nd "Table name" column to find the Journal lines table. Also, note that the "Reference table ID" value is not the same across different environments.
Another thing I learned - the +New button in document attachment isn't enabled until the General journal line is saved.
Always learning something new, no matter how long I've been at this!