private void StartUpLoad()

    {

        if (FileUpload1.HasFile)

        {

            HttpPostedFile postedFile = FileUpload1.PostedFile;

            if (IsImageFile(postedFile))

            {

                //Save image here

            }

            else

            {

                Response.Write("Invalid File, Cannot Upload!");

            }

        }

        else

        {

            Response.Write("Please select a File");

        }

    }

    protected bool IsImageFile(HttpPostedFile file)

    {

        bool isImage = false;

        System.IO.FileStream fs = new System.IO.FileStream(file.FileName, System.IO.FileMode.Open, System.IO.FileAccess.Read);

        System.IO.BinaryReader br = new System.IO.BinaryReader(fs);

        string fileclass = "";

        byte buffer;

        try

        {

            buffer = br.ReadByte();

            fileclass = buffer.ToString();

            buffer = br.ReadByte();

            fileclass += buffer.ToString();

        }

        catch

        {

            return false;

        }

        finally

        {

            br.Close();

            fs.Close();

        }

        /*extension lists with codes

         *7173        gif

         *255216      jpg

         *13780       png

         *6677        bmp

         *239187      txt,aspx,asp,sql

         *208207      xls.doc.ppt

         *6063        xml

         *6033        htm,html

         *4742        js

         *8075        xlsx,zip,pptx,mmap,zip

         *8297        rar  

         *01          accdb,mdb

         *7790        exe,dll          

         *64101       bat

         */

        //only allow images    jpg       gif     bmp     png     

        String[] fileType = { "255216", "7173", "6677", "13780" };

        for (int i = 0; i < fileType.Length; i++)

        {

            if (fileclass == fileType[i])

            {

                isImage = true;

                break;

            }

        }

        return isImage;

    }