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编辑：一些代码，以防原始源代码丢失

function wpse31877_add_custom_meta_boxes() 
{
    foreach ( array( \'post\', \'page\' ) as $post_type )
    {
        add_meta_box(
             \'wp_custom_attachment\'
            ,\'Custom Attachment\'
            ,\'wpse31877_custom_attachment\'
            ,$post_type
            ,\'side\'
        );
    }
}
add_action( \'add_meta_boxes\', \'wpse31877_add_custom_meta_boxes\' );

function wpse31877_custom_attachment() 
{
    // validate and secure our upload
    wp_nonce_field( plugin_basename(__FILE__), \'wpse31877_custom_attachment_nonce\' );

    printf( 
         \'<p class="description">%s</p>%s\'
        ,__( \'Upload your PDF here.\', \'your_custom_textdomain\' )
        ,\'<input type="file" id="wp_custom_attachment" name="wp_custom_attachment" value="" size="25">\'
);
}

function wpse31977_save_custom_meta_data( $id ) 
{
    // security verification
    if ( ! wp_verify_nonce( $_POST[\'wpse31977_custom_attachment_nonce\'], plugin_basename( __FILE__ ) ) )
    {
        return $id;
    }

    if ( defined( \'DOING_AUTOSAVE\' ) AND DOING_AUTOSAVE )
    {
        return $id;
    }

    if ( \'page\' === $_POST[\'post_type\'] )
    {
        if ( ! current_user_can( \'edit_page\', $id ) )
        {
            return $id;
        }
    }
    else
    {
        if ( ! current_user_can( \'edit_post\', $id ) )
        {
                return $id;
        }
    }

    // Make sure the file array isn\'t empty
    if ( ! empty( $_FILES[\'wp_custom_attachment\'][\'name\'] ) ) 
    {
        // Setup the array of supported file types. In this case, it\'s just PDF.
        $supported_types = array( \'application/pdf\' );

        // Get the file type of the upload
        $arr_file_type = wp_check_filetype( basename( $_FILES[\'wp_custom_attachment\'][\'name\'] ) );
        $uploaded_type = $arr_file_type[\'type\'];

        // Check if the type is supported. If not, throw an error.
        if ( in_array( $uploaded_type, $supported_types ) )
        {
            // Use the WordPress API to upload the file
            $upload = wp_upload_bits( $_FILES[\'wp_custom_attachment\'][\'name\'], null, file_get_contents( $_FILES[\'wp_custom_attachment\'][\'tmp_name\'] ) );

            if ( isset( $upload[\'error\'] ) AND 0 != $upload[\'error\'] )
            {
                wp_die( sprintf( __( \'There was an error uploading your file. The error is: %s\', \'your_textdomain\' ), $upload[\'error\'] ) );
            }
            else
            {
                add_post_meta( $id, \'wp_custom_attachment\', $upload );
                update_post_meta( $id, \'wp_custom_attachment\', $upload );       
            }

        }
        else
        {
            wp_die( __( "The file type that you\'ve uploaded is not a PDF.", \'your_textdomain\' );
        }

    }

} // end save_custom_meta_data
add_action( \'save_post\', \'wpse31877_save_custom_meta_data\' );

echo get_post_meta( get_the_ID(), \'wp_custom_attachment\', true );