Do you need to use Html.BeginForm()? You can simply do this with html instead HtmlHelper.
<form class="ng-pristine ng-valid" id="AppForm" action="" enctype="multipart/form-data" method="post" novalidate="novalidate">
</form>
If you need to use HtmlHelper, you can override the form action as follows:
@using(Html.BeginForm(null, null, FormMethod.Post, new { @action = "", enctype = "multipart/form-data", id = "AppForm" })) {
}