I have a html form, with a custom file upload field. And by that I mean that I have moved the actual file field beyond the borders of the page with css, that I have a custom
You need to change onsumbit='return false;' to onsubmit='return validateForm()'.
Then have validateForm() return true if the form passes your validation checks, or false if it does not.
The onsubmit='return false' is preventing the form from submitting via document.thisForm.submit(); as well as when the user clicks the submit button.