FormFile is an interface in Struts. So you would use a mock object to simulate it. You can write your own class to implement the interface. Or more likely, you will want to use a framework like jMock or easyMock to do this for you.
Although Jeanne is correct that you can use mocks in such a case, in my opinion they are rather deep in the bag of tricks, especially if you are rather new to JUnit.
So what I would look at beforehand is whether you really need to pass a FormFile object to that method. Can you perhaps abstract away from that Struts class and pass in an object that would make the test easier to write? What is the method doing you want to test?
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus