There is nothing messed up with the file, but there is something messed up with the server. The server sends the file with a text mime-tpye, so Safari adds the txt extension to the file. Get Info on the file in Finder and in the "Name & Extension" field of the Info dialog remove the .txt extension (you can not just simply remove it in Finder directly because that would just hide the extension). The file should then play fine with QuickTime Player.