A downflow furnace takes the air in at the top, warms it, and then blows it into your ductwork. The intake air is moving downward so the filter will be located at the top, in the upper blower compartment. Most attic furnaces are downflow furnaces. If it is in the attic or upstairs ...
Two local friends exchange witty banter even as they build custom ductwork for a current project house. This was a bigger job as the original 1910 house had no real ducting at all – we had to open up walls and run pipes to every room in the house. Your furnace upgrade will be easier...