WordPress/wp-includes/ms-files.php

<?php
/**
 * Multisite upload handler.
 *
 * @since 3.0.0
 *
 * @package WordPress
 * @subpackage Multisite
 */

define( 'SHORTINIT', true );
require_once( dirname( dirname( __FILE__ ) ) . '/wp-load.php' );

ms_file_constants();

error_reporting( 0 );

if ( $current_blog->archived == '1' || $current_blog->spam == '1' || $current_blog->deleted == '1' ) {
	status_header( 404 );
	die( '404 &#8212; File not found.' );
}

$file = BLOGUPLOADDIR . str_replace( '..', '', $_GET[ 'file' ] );
if ( !is_file( $file ) ) {
	status_header( 404 );
	die( '404 &#8212; File not found.' );
}

$mime = wp_check_filetype( $_SERVER[ 'REQUEST_URI' ] );
if( false === $mime[ 'type' ] && function_exists( 'mime_content_type' ) )
	$mime[ 'type' ] = mime_content_type( $file );

if( $mime[ 'type' ] )
	$mimetype = $mime[ 'type' ];
else
	$mimetype = 'image/' . substr( $_SERVER[ 'REQUEST_URI' ], strrpos( $_SERVER[ 'REQUEST_URI' ], '.' ) + 1 );

header( 'Content-type: ' . $mimetype ); // always send this
if ( false === strpos( $_SERVER['SERVER_SOFTWARE'], 'Microsoft-IIS' ) )
	header( 'Content-Length: ' . filesize( $file ) );

// Optional support for X-Sendfile and X-Accel-Redirect
if ( WPMU_ACCEL_REDIRECT ) {
	header( 'X-Accel-Redirect: ' . str_replace( WP_CONTENT_DIR, '', $file ) );
	exit;
} elseif ( WPMU_SENDFILE ) {
	header( 'X-Sendfile: ' . $file );
	exit;
}

$last_modified = gmdate( 'D, d M Y H:i:s', filemtime( $file ) );
$etag = '"' . md5( $last_modified ) . '"';
header( "Last-Modified: $last_modified GMT" );
header( 'ETag: ' . $etag );
header( 'Expires: ' . gmdate( 'D, d M Y H:i:s', time() + 100000000 ) . ' GMT' );

// Support for Conditional GET
$client_etag = isset( $_SERVER['HTTP_IF_NONE_MATCH'] ) ? stripslashes( $_SERVER['HTTP_IF_NONE_MATCH'] ) : false;

if( ! isset( $_SERVER['HTTP_IF_MODIFIED_SINCE'] ) )
	$_SERVER['HTTP_IF_MODIFIED_SINCE'] = false;

$client_last_modified = trim( $_SERVER['HTTP_IF_MODIFIED_SINCE'] );
// If string is empty, return 0. If not, attempt to parse into a timestamp
$client_modified_timestamp = $client_last_modified ? strtotime( $client_last_modified ) : 0;

// Make a timestamp for our most recent modification...
$modified_timestamp = strtotime($last_modified);

if ( ( $client_last_modified && $client_etag )
	? ( ( $client_modified_timestamp >= $modified_timestamp) && ( $client_etag == $etag ) )
	: ( ( $client_modified_timestamp >= $modified_timestamp) || ( $client_etag == $etag ) )
	) {
	status_header( 304 );
	exit;
}

// If we made it this far, just serve the file
readfile( $file );

?>