sendFormWithFiles(form_el, on_success, on_error = null, headers = {'Content-Type': 'multipart/form-data'}) {
        this.sendForm(form_el, on_success, on_error, headers);
    }
Sends a form via ajax POST with header Content-Type: multipart/form-data which is required for file uploading
Data is automatically taken form all form input values

@param {object} form_el - Form document element
@param {callback(responseText)} on_success - callback for status code 200
@param {callback(responseText, responseStatusCode)} on_error = null - custom handler for non 200 status codes
@param {object} headers = {'Content-Type': 'multipart/form-data'} - key: value map of headers