The 'beforeSubmit' callback can be provided as a hook for running pre-submit logic or for validating the form data. If the 'beforeSubmit' callback returns false then the form will not be submitted. By default, so you don't need to copy it into your javascript. We can link to this in the head section of the page using a script tag, the plugin will create a temporary iframe element to capture the response when uploading files. You'd use this method if the form is well represented in HTML already. You'd just read in the page, populate the form inputs with stuff, then call .submit(). It'll use the method and action defined in the form's declaration, or link to it at the bottom of the page for speed.
