I need an FTP client that doesn't make the to be overwritten file unusable while it is overwriting it. So it has to first for example upload the new file as a temp file and only then should it delete the old file and rename the temp file to become the target file.

Does anyone know such an FTP client that is otherwise good too?

I do not know.... I use Total Commander and just had a really serious problem with a file that I uploaded but something went wrong and after I overwrote it I found out that it wasn't the whole file... and I called the hosting company to ask them if they made a backup soon... it was a long and tiring situation...