How to check if the file is of specific type only.

byte[] EXE_SIGNATURE = {77, 90 };
private byte[] ReadTwoBytes(string filepath)
{
    try
    {
        using (FileStream fsSource = new FileStream(filepath, FileMode.Open, FileAccess.Read))
        {

            // Read two bytes from source file into a byte array.
            byte[] bytes = new byte[2];
            int n = fsSource.Read(bytes, 0, 2);
            return bytes;
        }
    }
    catch
    {

        return (byte[])null;
    }
}
//check exe signature
private bool IsExe(string filePath)
{
    byte[] b = ReadTwoBytes(filePath);
    return b.SequenceEqual(EXE_SIGNATURE);
}

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs)
       Label2.Visible = True
       Dim filePath As String = FileUpload1.PostedFile.FileName
       Dim filename1 As String = Path.GetFileName(filePath)
       Dim ext As String = Path.GetExtension(filename1)
       Dim type As String = [String].Empty

       If FileUpload1.HasFile Then

           Try


               Select Case ext
                   Case ".xls"

                       type = "application/vnd.ms-excel"

                       Exit Select

                   Case ".txt"
                       type = "application/vnd.txt"

                       Exit Select

               End Select

               If type <> [String].Empty Then
                   connection()
                   Dim fs As Stream = FileUpload1.PostedFile.InputStream
                   Dim br As New BinaryReader(fs)
                   Dim bytes As [Byte]() = br.ReadBytes(CType(fs.Length, Int32))
                   query = "insert into Excelfiledemo(Name,type,data)" & " values (@Name, @type, @Data)"
                   com = New SqlCommand(query, con)
                   com.Parameters.Add("@Name", SqlDbType.VarChar).Value = filename1
                   com.Parameters.Add("@type", SqlDbType.VarChar).Value = type
                   com.Parameters.Add("@Data", SqlDbType.Binary).Value = bytes
                   com.ExecuteNonQuery()
                   Label2.ForeColor = System.Drawing.Color.Green

                   Label2.Text = "File Uploaded Successfully"
               Else
                   Label2.ForeColor = System.Drawing.Color.Red


                   Label2.Text = "Select Only Excel and text Files "
               End If
           Catch ex As Exception


               Label2.Text = "Error: " & ex.Message.ToString()

           End Try
       End If
   End Sub