gistfile1.phtml

<?php
add_filter( 'sanitize_file_name', 'cb_sanitize_file_name' );
/**
 * {@internal Missing short description}}
 *
 * @link wp-includes|formatting.php
 * @todo causing errors at media-new.php
 */
function cb_sanitize_file_name( $filename ) {
  if ( isset( $_REQUEST['post_id'] ) )
    $post_id = $_REQUEST['post_id'];
  else
    $post_id = false;

  // fetch the file extension
  $info = pathinfo( $filename );
  $ext  = empty( $info['extension'] ) ? '' : '.' . $info['extension'];

  // get the post object
  $post_obj = get_post( $post_id );

  // get the post title
  $post_title = sanitize_title_with_dashes( $post_obj->post_title );

  // random number
  $random_number = rand( 10000, 99999 );

  return $post_title . '-' . $random_number . $ext;
}

I guess the add_filter call should be wrapped in an if-statement or a function that gets hooked on the needed page only using global $hook_suffix, $pagenow, $typenow or similar.