弹出页面中找不到定位器的元素

找不到Selenium这个元素:

我的机器人代码:

VA_M03_0108
[Tags]  Carregis
Login.Input user name   ${USER}         ${PASS}
SendCar
Click Element       id=ContentPlaceHolder1_rpt_search_lblCarReceiveDoc_0
Sleep  2s
Wait Until Element Is Visible     id=UpdatePanel1         timeout=20s
Choose File        id=MainContent_fuAttachment     ${CURDIR}/???????.jpg

机器人响应:

Element 'id=MainContent_fuAttachment' not visible after 20 seconds.

有人,请帮帮我.这是UI:https://www.sendspace.com/file/kqerdt