title: How to disable submit button until file is selected
body: <p>I have an HTML file upload page and I need the submit button to be disabled until a file is selected.  Then the submit button will be enabled and the user can upload.  I assume I can use jQuery code to query the file upload field's value to see whether a file has been selected to upload.  And I assume I can check that whenever focus has left the file upload field.  My concern is that some browsers might act funny with this and then my users can't upload any files.</p>
  
  <p>Is there a safe way to do this, preferably using jQuery, where I won't be in danger of having some users unable to upload?</p>