jQuery validate is a great plugin to help with form validation, however if your form uses file fields you may have found a quirky little "feature" that causes the file fields not to re-validate when the field changes.

After the form is submitted the file field is validated and the error message is shown as usual.
Despite choosing a file of an accepted mimetype the error is still visible.

To resolve this I had to manually re-validate the file field when it changed which will either add or remove the error message after the field changes as you would expect.