Fileupload with file type check

--------------------------------------
protected void btnUpload_Click(object sender, EventArgs e)
{
string path = Server.MapPath("upload/");
bool boolOk = false;

if (FUpload.HasFile)
{
string ext = String.Empty;
ext = System.IO.Path.GetExtension(FUpload.FileName).ToLower();

String[] allowed = { ".doc", ".pdf", ".txt" };
for (int i = 0; i <= allowed.Length - 1; i++)
{
if (ext == allowed[i])
{
boolOk = true;
}
if (boolOk == true)
{
try
{
FUpload.PostedFile.SaveAs(path + FUpload.FileName);
lblmsg.Text = "File uploaded successfully!!";
}
catch (Exception ex)
{
lblmsg.Text = ex.Message.ToString();
}

}
else
{
lblmsg.Text = "Only text files are supported. Please change!!";
}

}