Right right... but to go a step further, NO extension actually holds any meaning whatsoever. They are only used to identify files for applications to be associated with them. Once upon a time, we used to name files anything we wanted because we had so little storage space that it was easy to keep track of each of our files and we only had a handful of applications anyways...
Well... that's pretty close to true... For an example, take a PDF file - any one will do - rename it from myFile.PDF to myFile.fart. Open Acrobat and then click open. Browse to the file and open it. It will display correctly. You can even go so far as to rename it with a *.wetfart extension... and it won't even get messy at that point :)