NASes and so on have FTP/SFTP and other file transfer methods, but that's all they do- transfer files. They don't know context and policy on how long a file in a given folder should be kept. auto-file-deletion seems to me isn't an FTP server function. It's more like a user forum, blog or a video surveillance archive app as exists in some NASes as below.
The video archiver in Synology (and probably QNAP and others as well) can be setup to constrain the archive size by deleting old files. You can read about that in the vendors' user manual.
The time backup capability in the afforementioned NASes will delete file cersions older than X weeks, but you probably want the video surveillance/archiving system meet your needs.
They synology I have is typical of such.. it takes an IP video stream from an IP camera (not via FTP), or a single-frame still shot every x seconds, or a stream/frame when motion is detected, etc., and archives. The size of the archive is your choice. Most cameras I've seen can be an FTP client and send that way, but its not common, vs. the taking the camera's native format. The NASes have to support a lot of proprietary camera formats.... and they do, as they sell to small business and so on that have multi-camera surveillance systems with forensics and so on.
Last edited by stevech; 02-22-2013 at 02:06 AM.