I know this is old but you could create a custom extension if you needed to create that form over and over:
public static MvcForm BeginMultipartForm(this HtmlHelper htmlHelper)
{
return htmlHelper.BeginForm(null, null, FormMethod.Post,
new Dictionary<string, object>() { { "enctype", "multipart/form-data" } });
}
Usage then just becomes
<% using(Html.BeginMultipartForm()) { %>