AjaxControlToolkit Asyncfileupload with master page not working

I am using Ajax AsyncFileUpload control in asp.net 4.0 with master page and I get error that file is invalid. If I use it without master page, it works. With master page If I set

ClientIDMode="AutoID"

It start working. What is purpose of ClientIDMode and why AsyncFileupload is not working without ClientMode w开发者_StackOverflow社区ith master page.

Please suggest.

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The ClientID value is generated by concatenating the ID values of each parent naming container with the ID value of the control. In data-binding scenarios where multiple instances of a control are rendered, an incrementing value is inserted in front of the control's ID value. Each segment is separated by an underscore character (_). This algorithm was used in versions of ASP.NET earlier than ASP.NET 4.

From MSDN

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The client side script got confused because it couldn’t find the predicted ClientID.