[tvjames]

I've almost exclusively seen this handled using an upload with a temporary filename (usually convention based) and then renamed on completion. Watch then for the existence of the renamed files. Clean up temp files after a defined amount to time (30 days for example).

[skrebbel]

Ahyes, smart. We didn't control the client, unfortunately, so we couldn't do that.

(we tried to communicate with the people who did but they turned out to be the supplier to the supplier to the customer of our customer, 5 countries away, and fixing anything at all was not high on their priority list :D)

[NicoJuicy]

Afterwards change the extension to .xmlok or .xmlfail (alt: zip it), no need for vendor adjustments

[NicoJuicy]

Did that to, even open sourced a client to remove folders depending extension and x days https://github.com/NicoJuicy/CLI-Cleanup-IO

Ftp was because of legacy though, I would have preferred http post