Clear the label messages at the time of choosing the file using file uploader

you need to add onchange on to fileupload

<asp:FileUpload ID="FileUpload1" runat="server"  onchange="callme();" />

and then, create a JS function to clear your messages

function callme() {
         document.getElementById("your message label id").value= "";
       }

I didn't test, but this should do what you want.