File Upload in Update Panel[
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You can refer this link, and below info may help you to stop refreshing entire page
Tag
Description
<asp:asyncpostbacktrigger xmlns:asp="#unknown">
Specifies a control and event that will cause a partial page update for the UpdatePanel that contains this trigger reference.
<asp:postbacktrigger>
Specifies a control and event that will cause a full page update (a full page refresh). This tag can be used to force a full refresh when a control would otherwise trigger partial rendering.