AIUI, basically it means that whilst a program written in, say, python, can itself by copyrightable; the python language itself can't be. This means you, or anyone else, can implement your own python interpreter without any fear of being accused of copyright infringment.
Seems like you could extend this same argument to file by arguing you are writing an interpreter for the PDF/GIF/AutoCAD/PSD notation.
From the point of defending against copyright violation, you can. However, patents and reverse engineering laws present separate barriers you could need to jump over (although this is no longer a problem for at least 3 of the 4 formats you mention, I believe).