This is speculation, but it could be a consequence of not wanting to lock the entire file. Where a normal application could place locks on segments of a file, a

This is speculation, but it could be a consequence of not wanting to lock the entire file. Where a normal application could place locks on segments of a file, a web app perhaps can't do this through this api. Hence you have the choice between aborting on write and possible "garbage files" (i.e. partly overwritten) if you keep reading.