WordPress at a glance
filter is not described

wp_count_attachments filter-hook . WP 3.7.0

Modify returned attachment counts by mime type.

Usage

add_filter( 'wp_count_attachments', 'filter_function_name_6612', 10, 2 );
function filter_function_name_6612( $counts, $mime_type ){
	// filter...

	return $counts;
}
$counts(object)
An object containing the attachment counts by mime type.
$mime_type(string)
The mime type pattern used to filter the attachments counted.

Where the hook is called

wp_count_attachments()
wp_count_attachments
wp-includes/post.php 2465
return apply_filters( 'wp_count_attachments', (object) $counts, $mime_type );

Where the hook is used (in WP core)

Does not used.