I will rephrase what @F-ES Sitecore probably intended to share:
FileUpload control is not supported when working along partial update as it requires a full page postback.
What does this translates to, two options:
1. Use a PostbackTrigger on UpdatePanel to make it happen
<asp:UpdatePanel ID="FormUpdatePanel" runat="server">
<ContentTemplate>
<div>
<asp:FileUpload ID="EntryFile1Upload" runat="server" />
<br />
<asp:Button ID="UploadButton" OnClick="UploadButton_Click" runat="server" Text="Upload File" />
</div>
</ContentTemplate>
<Triggers>
<asp:PostBackTrigger ControlID="UploadButton" />
</Triggers>
</asp:UpdatePanel>
Refer:
https://www.surinderbhomra.com/Blog/2018/12/18/Getting-FileUpload-To-Work-Inside-An-UpdatePanel[
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2. Use another control that supports async file upload.
In past, I have used following:
AsyncFileUpload Sample[
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Also, found a CP article on similar control:
Asynchronous File Upload[
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Give a try.