Bug 1093874 - new test case for loading various different image file format

This test loads a mix of valid / invalid image file formats, and verifies whether the app still functions normally.
Shouldn't this be:
back_button = Wait(self.marionette, timeout=60).until(
    expected.element_present(*self._fullscreen_back_button_locator))
?
Or is this element assured to be there?