Networks and Sites: Add support for file/upload type inputs on the "New Site" screen.

This change allows plugins to add files when creating sites via the network-admin "New Site" form.

* Adds `enctype="multipart/form-data"` to the form in `wp-admin/network/site-new.php`.
* Supports `input type="file"` fields in this form (via the `network_site_new_form` action).

Props johnjamesjacoby, sakibmoon, sirlouen, realloc.

Fixes #62086.

git-svn-id: https://develop.svn.wordpress.org/trunk@60398 602fd350-edb4-49c9-b593-d223f7449a82

Author: JJJ

src/wp-admin/network/site-new.php

@@ -204,7 +204,7 @@
 }
 ?>
 <p><?php echo wp_required_field_message(); ?></p>
-<form method="post" action="<?php echo esc_url( network_admin_url( 'site-new.php?action=add-site' ) ); ?>" novalidate="novalidate">
+<form method="post" enctype="multipart/form-data" action="<?php echo esc_url( network_admin_url( 'site-new.php?action=add-site' ) ); ?>" novalidate="novalidate">
 <?php wp_nonce_field( 'add-blog', '_wpnonce_add-blog' ); ?>
 	<table class="form-table" role="presentation">
 		<tr class="form-field form-required">