Thanks for your help jpn, but the file isn't created on my computer.
Perhaps there is connection error, due to the proxy or the port.
I'll try to check it out .

In my previous post I said the file was created, but it was created by a simple fileOpen() .
I used Ethereal to analyze my network, and I noted that no HTTP packet were emitted.

Now I understand better what happens, and why everybody says me : "Don't use post(), use request() !". The post() don't allow me to set my own headers like it is necessary

Thanks for all, I'll try to do it