function wp_unique_filename( $dir, $filename, $unique_filename_callback = null )
======
function wp_upload_bits( $name, $deprecated, $bits, $time = null ) {
if ( empty( $name ) )
return array( 'error' => __( 'Empty filename' ) );
$wp_filetype = wp_check_filetype( $name );
if ( !$wp_filetype['ext'] )
return array( 'error' => __( 'Invalid file type' ) );
$upload = wp_upload_dir( $time );
if ( $upload['error'] !== false )
return $upload;
***=> $filename = wp_unique_filename( $upload['path'], $name );
There is no way to use custom wp_unique_filename generator @ function wp_upload_bits