fileupload via ftp and attachment_fu

How can I use attachment_fu to store files in a MySQL database (db_file) which I uploaded
via ftp in a special directory?

I need to upload files - large images 25-40 MB -, so my intention ist to upload them
via ftp and then use attachment_fu to strore them in a MySQL database.

I had problems in uploading such files via a form - timeouts.

I need them in a database because we have a very complicated rights-matrix for
accessing this files.