Hi,
There's no built-in media manager in Acymailing. It just uploads you attachments to a folder outside your upload folder, so you can't access it with your CMS media manager. You can delete these files using a ftp client.
If you wish you can change the folder for your attachements in Configuration->Security. If you choose your upload folder you can access your files with the media manager afterwards. Please not that changing the folder does not move your existing files.
