Unit testing code with a file system dependency

I am writing a component that, given a ZIP file, needs to:

1. Unzip the file.
2. Find a specific dll among the unzipped files.
3. Load that dll through reflection and invoke a method on it.

I'd like to unit test this component.

I'm tempted to write code that deals directly with the file system:

void DoIt()
{
   Zip.Unzip(theZipFile, "C:\\foo\\Unzipped");
   System.IO.File myDll = File.Open("C:\\foo\\Unzipped\\SuperSecret.bar");
   myDll.InvokeSomeSpecialMethod();
}

But folks often say, "Don't write unit tests that rely on the file system, database, network, etc."

If I were to write this in a unit-test friendly way, I suppose it would look like this:

void DoIt(IZipper zipper, IFileSystem fileSystem, IDllRunner runner)
{
   string path = zipper.Unzip(theZipFile);
   IFakeFile file = fileSystem.Open(path);
   runner.Run(file);
}

Yay! Now it's testable; I can feed in test doubles (mocks) to the DoIt method. But at what cost? I've now had to define 3 new interfaces just to make this testable. And what, exactly, am I testing? I'm testing that my DoIt function properly interacts with its dependencies. It doesn't test that the zip file was unzipped properly, etc.

It doesn't feel like I'm testing functionality anymore. It feels like I'm just testing class interactions.

My question is this: what's the proper way to unit test something that is dependent on the file system?

edit I'm using .NET, but the concept could apply Java or native code too.