FTP is one of the oldest protocols still in daily use, and file transfer as a problem has outlived it several times over. Managed file transfer is a real enterprise software category with compliance requirements and serious budgets behind it.
FTPX reads as the next thing after FTP, which is a useful position. The X suffix signals extension or successor without claiming to be a specific standard, and anyone technical reads it immediately.
The name fits a file transfer product, a managed transfer service, an open protocol project, a standards initiative, or a technical resource. On the .org, the open project and standards readings come forward, which suits an alternative to the proprietary enterprise tools that dominate the space.
Four characters, no hyphen, entirely legible to the technical audience, and short enough to live comfortably in documentation, config files, and command lines.
Enquiries welcome.