[Plugin: Document Attachment Widget] filesize function fails in WordPress Multisite

It works on the main site but when used as a widget in subdomains this is what I get:

Warning: filesize() [function.filesize]: stat failed for www.domain.com/htdocs/wp-content/plugins/document-attachment-widget/includes/determine_file_size.php on line 14

http://wordpress.org/extend/plugins/document-attachment-widget/