Do we need 100% code coverage when it comes to unit testing?

We are trying to do more TDD but I've come across a stumbling block. We are using PHPUnit+FuelPHP and I need/should write a unit test for image uploads.

However, mocking anything dealing with the FILESYSTEM is notoriously difficult and time consuming.

So I am wondering: Does it make sense to write a unit test when writing testing code (i.e. mocking and stubbing and writing a bunch of wrappers) is more involved than writing actual production code?

Do we need 100% test coverage? Or am I approaching TDD incorrectly?