ASP.Net Handler Request.Files总是空的

I'm writing a C# ASP.Net application for client to post files to other server. I'm using a generic handler to handle posted files from client to server. But in my handler, context.Request.Files always empty (0 count). I believe my post method is right, because when I tried to move the handler in the same domain as the client, I can accept the files and save them. But the problem is I need to save the files to the other server.I'm writing a C# ASP.Net application for client