File Upload data getting cleared after a textbox changed event fired

Question

 <td style="border: 1px solid #1a1a1a; padding: 5px 8px;" align="center">
                                    <asp:HiddenField ID="hdnFilePath1" runat="server" />
                                    <asp:HiddenField ID="hdnFilename1" runat="server" />
                                    <asp:HiddenField ID="hdnFileUniqueName1" runat="server" />
                                    <asp:FileUpload ID="fuquationFile1" runat="server" AllowMultiple="true" />

                                    <asp:RequiredFieldValidator ID="RequiredFieldValidator25" runat="server" Text="*" ControlToValidate="txtOfferedPrice_P1"
                                        ErrorMessage="*" SetFocusOnError="true" ValidationGroup="ValDept" InitialValue="">
                                    </asp:RequiredFieldValidator>
                                </td>

 

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    <asp:TextBox ID="txtBasicValueGst18Per" runat="server" onkeypress="return alertBox(this,event);" autocomplete="off"
                    TextMode="SingleLine" TabIndex="6" Width="150" MaxLength="10" AutoPostBack="true" CssClass="form-control" OnTextChanged="txtBasicValueGst18Per_TextChanged"></asp:TextBox>