Could you try with below piece of code -
If you are using "AS" operator you dont need to user explicit casting while finding control.
FileUpload newfileuploadcontrol = FormView1.FindControl("FileUpload1") as FileUpload;
TextBox newtextbox = FormView1.FindControl("TextBox10") as TextBox;
if (newfileuploadcontrol.HasFile)
{
string path = Server.MapPath("/Cars/Copy_of_Payment_Images/") + newfileuploadcontrol.FileName;
newfileuploadcontrol.SaveAs(path);
newtextbox.Text = path;
}