That file is currently on the server.
I did a quick test by manually uploading 2 files (1 of 40MB, 1 of 80MB) and they were visible on the server right at the end of the transfer (ie: maximum within 1 second 
).
However, during the upload the file isn’t visible in the directory, while on another server it is, so it may have a few millisecond delay for it to appear in the listing and it’d make Duplicati not happy…?