In a stockpot or large saucepan, heat the bacon fat. Add the ham hock and onions and cook for 10 minutes, until onions are softened and slits in ham hock have begun to open up. Add garlic, bay leaves, thyme, and cayenne pepper and cook for 2 minutes, stirring occasionally. Add chopped greens, parsley, green onions and cabbage and cook for 10 to 15 minutes, stirring frequently, until greens are wilted. Add broth or water and increase heat to high. Bring to a boil, skim surface, and reduce heat to a simmer. Cook for 2 1/2 hours, until greens are very tender. Season with salt and pepper and add file, to taste, just before serving in large bowls over hot white rice. (Do not allow gumbo to boil after adding the file.)