The FTP source option only works with the Import directory of your Enhanced FTP account, not with any other FTP account.
One solution would be that if your external FTP account was bound to a published web directory (that is, any files in the directory could be publicly available from an HTTP/HTTPS URL), then you could use the
HTTP option in the AttachFile() function.
If security was a concern (that these files are accessible from a publicly available URL), then there are a couple of options:
1. Basic access authentication
Password protect access to the publicly available web directory and the containing files (this is a default feature in IIS and Apache) then use basic access authentication in conjunction with an HTTPS endpoint, for example:
Note that as the connection is over HTTPS, the username and password credentials are secure and can't be compromised in transit.
2. IP Address Restriction
Restrict access to the published web directory (again, a standard feature in most web servers) and only allow ExactTarget IP addresses. The IP addresses will vary depending on which stack you are on, but you can review the complete list here.