Hi,
Although i'm not answering your question directly i propose two other approaches to modify forms.
1. Directly within designer i.e. you may even copy paste the content you have prepared. It is a small icon that appears when you select and hover a human task on your bpmn diagram, that looks like a person. If you click it the code of the ftl is presented and rendered real time as you edit it.
2. If you go within guvnor to the assets of your package i.e. defaultPackage, and select "open" for the specific form, a page is shown with download and upload fields.