I am pretty sure you could have problems. Try this:
Start a upload of a large file to your FTP server.
Refresh the view of the folder it is being uploaded to and you will see that the file size increases and the upload progresses.
If you try this with a .mp3 file. You can access it via the browser and see that it will only play up to the point that is has currently been uploaded.
This is why programs like Dreamweaver have a check in/check out system. So that if someone is working on a .html file someone else cannot upload a older version or cause that sort of problems.
I don't think FTP uses any kind of temp files or queuing either...
Correct. I once unzipped a file that actually was not transferred all the way. Weird error messages, of course... – Arjan – 2009-08-20T12:23:02.367
I think updating and existing file is different from creating a new file (the read will not start before the create happens, and then it will follow the write). If the read races-beyond the write, you get incompletely terminated reads. An incomplete ZIP file will show corruption. An incomplete mp3 will play up to the first point of corruption (i think). That is a difference in the file-formats. – nik – 2009-08-20T13:28:09.917
A modify/overwrite of an existing file should identify a read-in-progress and create a new version-to-write, retaining the older one for the read to complete. That is why I say in my answer that a typical filesystem would give an older copy of the file in such a case. – nik – 2009-08-20T13:32:59.597