jQuery Validate File Fields Quirkiness - Example of how to manually update the valid state of a file field when using jQuery Validate.

validate.js

// jQuery Validate setup.
$('form').validate({
  rules: {
    "cover-letter": {
      required: true,
      accept: "application/pdf|application/msword|application/vnd.openxmlformats-officedocument.wordprocessingml.document"
    }
  },
  messages: {
    "cover-letter": "Please upload your cover letter (PDF, DOC or DOCX)."
  }
});

// The fix for file fields not re-validating when they change.
$('#cover-letter').change(function () { $(this).valid(); });