Validate extension for dynamic FileUpload control in ASP.Net

Hi sam34567,

Please refer below sample.

HTML

<div>
    <asp:Label ID="lblHtml" runat="server" />
</div>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script type="text/javascript">
    function ValidateExtension(ele) {
        var allowedFiles = [".jpg", ".jpeg"];
        var fileUpload = $(ele);
        var lblError = $(ele).closest('tr').find('[id*=lblError]');
        var regex = new RegExp("([a-zA-Z0-9\s_\\.\-:])+(" + allowedFiles.join('|') + ")$");
        if (!regex.test(fileUpload[0].value.toLowerCase())) {
            lblError[0].innerHTML = "Please upload files having extensions: <b>" + allowedFiles.join(', ') + "</b> only.";
            return false;
        }
        lblError[0].innerHTML = "";
        return true;
    }
</script>

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    string file = "";
    for (int counter = 1; counter < 3; counter++)
    {
        file += "<div><table id='tbl'><tr><td><input type='file' id='fileUpload' accept='.jpg,.jpeg' onchange='return ValidateExtension(this)' /><span id='lblError' style='color: red;'></span><br /><td> <input type='submit' id='btnUpload' value='Upload' /></td></td></tr></table></div>";
    }
    lblHtml.Text = file;
}

Screenshot