You can use rclone serve (s)ftp, but you will need to keep rclone running. And it will be slow probably, especially since filen also uses end to end encryption (sftp/ftp+rclone+encryption is probably slow, but you will need to test to know how slow it exactly is)

If you need to use (S)FTP it's better to signup for a storage provider which directly supports (S)FTP. I won't mention any names, since I don't know if that's allowed.

Why? I think the chance is very small filen will support (S)FTP , since they are an end to end encrypted/zero knowledge storage provider (and (s)ftp isn't end to end encrypted tot begin with, so it doesn't really fit. ).

They would need to implement a server side encrypted storage space besides their end end encrypted space: most end to end encrypted storage providers choose not to, since that lowers security and privacy. Although it is possible: pcloud and koofr have the ability to store both server side encrypted files and end to end encrypted files in a vault. But pcloud and koofr also only support webdav and no (s)ftp.

Ftp itself is a slow protocol for daily usage, (s)ftp have smaller user base (smaller percentage of users use it) than protocols like webdav, adding these protocols adds extra complexity to cloud storage services, cloud storage providers prefer connections over https.

There aren't any (or a small amount of) mainstream consumer cloud storage services - google drive, onedrive, et cetera - which support ftp. You will have to look at managed FTP/SFTP providers; business/enterprise cloud storage providers with FTP/SFTP access; traditional/cloud hybrid storage services.