WordPress at a glance
filter is not described

upload_per_page filter-hook . WP 2.9.0

Filters the number of items to list per page when listing media items.

Usage

add_filter( 'upload_per_page', 'filter_function_name_8650' );
function filter_function_name_8650( $media_per_page ){
	// filter...

	return $media_per_page;
}
$media_per_page(int)
Number of media to list.
Default: 20

Where the hook is called

wp_edit_attachments_query_vars()
wp-admin/includes/post.php 1153
$q['posts_per_page'] = apply_filters( 'upload_per_page', $media_per_page );

Where the hook is used (in WP core)

Does not used.