title: What is a good way to test a file to see if its a zip file?
body: <p>I am looking as a new file format specification and the specification says the file can be either xml based or a zip file containing an xml file and other files.</p>
  
  <p>The file extension is the same in both cases.  What ways could I test the file to decide if it needs decompressing or just reading?</p>