How to make “Upload files”selected by default in Insert Media?

Add this to your functions.php, or preferably a functionality plugin.

add_action( 'admin_footer-post-new.php', 'media_manager_default' );
add_action( 'admin_footer-post.php', 'media_manager_default' );

function media_manager_default() {
    ?>
    <script type="text/javascript">
        jQuery(document).ready(function($){
            wp.media.controller.Library.prototype.defaults.contentUserSetting=false;
        });
    </script>
    <?php
}