Pop up a media modal to upload/select an XML file. This assumes specific HTML output in attach_xml_box (the metabox callback). Use the function in enqueue.php to load custom-media.js & the files required for media on the hompage edit screen.

custom-media.js

// Uploading files
var file_frame;

( function( $ ) {

$(document).ready( function(){

  var $container = $( document.getElementById( 'attach-xml' ) );

	$container.on( 'click', '.upload', function( event ){
		event.preventDefault();
		var $this = $( this );
	
		// If the media frame already exists, reopen it.
		if ( file_frame ) {
			file_frame.open();
			return;
		}

		// Create the media frame.
		file_frame = wp.media.frames.file_frame = wp.media({
			title: $this.data( 'title' ),
			button: {
				text: $this.data( 'select' ),
			},
      // Specify only text files, since we want only XML.
			library: {
				type: 'text'
			},
			multiple: false
		});

		// When an image is selected, run a callback.
		file_frame.on( 'select', function() {
			// We set multiple to false so only get one image from the uploader
			attachment = file_frame.state().get('selection').first().toJSON();
	
			// Do something with attachment.id and/or attachment.url here
			if ( 'text' == attachment.type ){
				$container.find( '[name="xml_post_id"]' ).val( attachment.id );
				$container.find( '.display' ).html( attachment.title );
				$container.find( '.remove' ).show();
				$container.find( '.upload' ).hide();
			}
		});

		// Finally, open the modal
		file_frame.open();
	});
	
	$container.on( 'click', '.remove', function(){
		$container.find( '[name="xml_post_id"]' ).val( '' );
		$container.find( '.display' ).html( '<i>none selected</i>' );
		$container.find( '.upload' ).show();
		$container.find( '.remove' ).hide();
	});

} );

} )( jQuery );

enqueue.php

<?php
    //[... inside a class]
    public function add_media( $screen ) {
		if ( 'post.php' == $screen && 'homepage' == get_post_type() ) {
			wp_enqueue_media();
			wp_enqueue_script( 'wired-admin', get_template_directory_uri() . '/custom-media.js', array( 'jquery' ) );
		}
	}

    add_action( 'admin_enqueue_scripts', array( $this, 'add_media' ) );