All form data has be sent to the server, that includes the uploaded files, before the server can process the form and perform the validation.
It is not possible for the server to validate the form until the form data is sent.
one problem i see on your page is that you have 2 instances of the Server validation, one for the rest of the form and another for the captcha.
you remove the second instance, then edit the first instance to add the captcha validation.