title: Valid zip file in python
body: <p>I uploaded zip folder using file input,but when i check if this zip is valid or not;the result was:is not a valid pkzip file,i don't know what is the specification for valid pkzip.
  I used this code to check:</p>
  
  <pre><code>form = cgi.FieldStorage() 
  file_upload = form['file[]']
  if zipfile.is_zipfile(file_upload.filename):
      print "%s is a valid pkzip file" % file_upload.filename
  else:
      print "%s is not a valid pkzip file" % file_upload.filename
  </code></pre>
  
  <p>I don't know why?for example when i upload test.zip,the o/p will be:
  test.zip is not a valid pkzip file,I need a help.Thanks</p>