img: trim file path state var down to filename
img: trim file path state var down to filename

Calculate filename right in _load_from_file() rather than carrying the
whole file path as object state.

kevingu1003/python-pptx,cchanrhiza/python-pptx,scanny/python-pptx,hoopes/python-pptx,biggihs/python-pptx,AlexMooney/python-pptx