Why does AsyncFileUpload provide postback?

I use AjaxControlToolkit AsyncFileUpload to upload files to the server. It is placed in the UpdatePanel, but when someone uploads a new file, a message appears. Why does the backward message happen when I put AsyncFileUpload in the UpdatePanel?

I used a sample form here and here .

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Source: https://habr.com/ru/post/1728734/


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