Where a folder refuses the temporary file or the rename, the new content
(whole and flushed elsewhere) is copied into the file itself: the file was
truncated first, so a copy cut short by a lost connection or a crash left
neither version in place, and only the new one on disk under a hidden name.
The file as it was is now copied and flushed beside the temporary file before
it is truncated; if it can't be, the file is left alone and the save fails,
naming the temporary file. A failed copy names both.