Code to upload Image and signature Coupled with its Validations

//checks if it is an image
                private bool IsImage(byte[] data)
                {
                        //read 64 bytes of the stream only to determine the type
                        string myStr = System.Text.Encoding.ASCII.GetString(data).Substring(0,16);
                        //check if its definately an image.
                        if(myStr.Substring(8,2).ToString().ToLower()!="if")
                        {
                                //its not a jpeg
                                if(myStr.Substring(0,3).ToString().ToLower() != "gif")
                                {
                                        //its not a gif
                                        if(myStr.Substring(0,2).ToString().ToLower() != "bm")
                                        {
                                                //its not a .bmp
                                                if(myStr.Substring(0,2).ToString().ToLower() != "ii")
                                                {
                                                        //its not a tiff
                                                        //ProcessErrors("notImage");
                                                        this.myFile.PostedFile.InputStream.Close();
                                                        myStr = null;
                                                        return false;
                                                }
                                        }
                                }
                        }
                        myStr = null;
                        return true;
                }

<script language="javascript"> 
     function ValidateFile(source, args) 
     { 
        try 
         {        
        var fileAndPath= 
           document.getElementById(source.controltovalidate).value; 
        var lastPathDelimiter=fileAndPath.lastIndexOf("\\"); 
        var fileNameOnly=fileAndPath.substring(lastPathDelimiter+1);        
        var file_extDelimiter=fileNameOnly.lastIndexOf("."); 
        var file_ext=fileNameOnly.substring(file_extDelimiter+1).toLowerCase(); 
        if(file_ext!="jpg") 
             { 
             args.IsValid = false; 
             if(file_ext!="gif") 
               args.IsValid = false; 
                  if(file_ext!="png") 
                  { 
                    args.IsValid = false; 
                     return; 
                  } 
                } 
        }catch(err) 
        { 
          txt="There was an error on this page.\n\n"; 
          txt+="Error description: " + err.description + "\n\n"; 
          txt+="Click OK to continue.\n\n"; 
          txt+=document.getElementById(source.controltovalidate).value; 
          alert(txt); 
          } 
          
           args.IsValid = true; 
    } 
    </script>

<asp:fileupload id="FileUpload1" runat="server" xmlns:asp="#unknown" /> 
 
<asp:customvalidator id="CustomValidator1" xmlns:asp="#unknown">
ClientValidationFunction="ValidateFile"  runat="server"  
ControlToValidate="FileUpload1"  
Display="dynamic" ErrorMessage="images only ">