The answer will depend on the o/s and how it's been tuned. I suspect (suspect is one step above wild-assed guess) that a file that's being overwritten won't be updated until the write operation is completed, so that a communication failure won't leave a corrupted file. For a new file, the size and modification information will be updated when the write-buffer is flushed.

The answer will depend on the o/s, how it's been tuned, and the details of how the file transfer protocol is managed at the receiving end.