Would be great to have the ability to enable a sync file extension or adding an in progress flag of some sort to files that are currently being synced to alert other programs to not process such a file.
Other sync software uses !sync, lftp-pget-status, part, bts, !qb as examples.
That adds extra API hits for providers that care about it, it's a very massive change to make and I'm not a fan of adding more overhead to a very clean process.
I should have said, that would only be for backends that can upload partial files, like local, ftp, and sftp - the others wouldn't change as there is no point as you said.