How to FindControl a child element

I have a Div element and a FileUpload control under the Div.
I can't seem to FindControl the FileUpload element, althougth I can see that the Div has the control inside it.
This is my code:
wUpload = (System.Web.UI.WebControls.FileUpload)wdiv.FindControl("W_16_W1");
This is what I see in the Watch window:
wdiv.Controls[1].ID --> "W_16_W1"
(System.Web.UI.WebControls.FileUpload)wdiv.FindControl("W_16_W1") --> null