Is there a way to upload a file to a FTP server when behind an HTTP proxy ?
It seems that uploading a file is not supported behind an HTTP Proxy using .Net Webclient
As Alexander says, HTTP proxies can proxy arbitrary traffic. What you need is an FTP Client that has support for using a HTTP Proxy. Alexander is also correct that this would only work in passive mode.
My employer sells such an FTP client, but it is a enterprise level tool that only comes as part of a very large system.
I'm certain that there are others available that would better fit your needs.