my quick time player wont play any videos i download from youtube.why is that,i download an update of it and it still dont work.!

My quicktime player wont play any videos i download from youtube,when i double click the message comes up"the document flo rida -wild ones ft-1.mp4" could not be open.the movies file format isnt recongnised.To see if additional software is available that will enable quicktime player to open the movie,click tell me more." I tried that and even download an update,still does not work.pls help!