WordPress at a glance
filter is not described

upload_dir filter-hook . WP 2.0.0

Filters the uploads directory data.

Usage

add_filter( 'upload_dir', 'filter_function_name_3391' );
function filter_function_name_3391( $uploads ){
	// filter...

	return $uploads;
}
$uploads(array)
Array of upload directory data with keys of 'path', 'url', 'subdir, 'basedir', and 'error'.

Where the hook is called

wp_upload_dir()
upload_dir
wp-includes/functions.php 1909
$uploads = apply_filters( 'upload_dir', $cache[ $key ] );

Where the hook is used (in WP core)

Does not used.