Session overrighting problem during file uploading ?

below is the code..

i call it with a javascript funtion for google like upload u=in which it start uploading just after file selection using fileuploader.js file..



public string UploadSign(HttpPostedFileBase uploadFile, Int64 candid)
{
//List<circularfilemodel> myfile = new List<circularfilemodel>();
Int64 candidateid = 0;
string examType = "", status = "", challanNo = "";

try
{
candidateid = Convert.ToInt64(Session["CandidateID"]);
examType = Convert.ToString(Session["ExamType"]);
}
catch (Exception ex)
{

}

//string challanNo = "0";
bool isContup = false;

challanNo = candidateid.ToString() + "_" + examType;
isContup = true;

if (Request.ContentLength > 0 && isContup == true && candidateid >= 1 && candidateid == candid)
{
//CircularFileModel f1 = new CircularFileModel();

DateTime tm = DateTime.Now;
//string filename = Request.QueryString[0];
string filename = Request.QueryString["qqfile"];
string dirPath = AppDomain.CurrentDomain.BaseDirectory;
string fileURl = "./Content/writereaddata/Annual2015/";
string fileURl2 = "Content/writereaddata/Annual2015/";
if (Directory.Exists(dirPath + fileURl) == false)
{
Directory.CreateDirectory(dirPath + fileURl);
}
string cirfileName = challanNo.ToString() + "_Sign";
//string cirextn = Path.GetExtension(filename);
string cirextn = "jpg";

try
{
String fn = Path.GetFileNameWithoutExtension(filename);
String ext = Path.GetExtension(filename);
char[] SpecialChars = "!@#$%^&*()+=~`\\|/?><,\"".ToCharArray();
int indexOf = fn.IndexOfAny(SpecialChars);
String fileName1 = fn;
int count = fileName1.Split('.').Length - 1;
if (count > 1)
{
//return "Double extension not allowed in ";
return "{success:false, imsg:\"Double extension are not allowed in File-Name, please rename and try again.\"}";
}
if (indexOf != -1)
{
//return "Special character not allowed in ";
return "{success:false, imsg:\"Special character are not allowed in File-Name, please rename and try again.\"}";
}

string mimetype = Request.ContentType;
if (mimetype == "image/jpeg" || mimetype == "image/jpg" || mimetype == "image/pjpeg"
|| mimetype == "application/jpg" || mimetype == "application/x-jpg" || mimetype == "image/pipeg" || mimetype == "image/vnd.swiftview-jpeg"
|| mimetype == "image/x-xbitmap" || mimetype == "application/octet-stream")
{
if ((ext.ToLower() == ".jpg") || (ext.ToLower() == ".jpeg"))
{
fn = "";

string fFullName = filename;
int len = fFullName.Length;
string ext1 = Path.GetExtension(fFullName);
string str = fFullName.Substring(fFullName.LastIndexOf("\\") + 1);
len = str.Length;
string fileN = str.Substring(0, len - ext1.Length);

Regex FilenameRegex = null;
FilenameRegex = new Regex("(.*?)\\.(jpeg|jpg|JPEG|JPG)$", RegexOptions.IgnoreCase);
int index = fileN.IndexOf(".");

//if ((firstLine.IndexOf("JFIF") > -1) || (firstLine.IndexOf("Exif") > -1))
//{
if (Request.ContentLength <= 300 * 1024)
{
//if (System.IO.File.Exists(dirPath + fileURl + cirfileName + "." + cirextn))
//{
// System.IO.File.Delete(dirPath + fileURl + cirfileName + "." + cirextn);
//}

//uploadedfile.SaveAs(context.Server.MapPath(dirPath + "/" + fileSave));

//byte[] sr = context.Request.BinaryRead(context.Request.ContentLength);

fs.Seek(0, SeekOrigin.Begin);

Stream inputStream = Request.InputStream;

if (System.IO.File.Exists(dirPath + fileURl + cirfileName + "." + cirextn))
{
System.IO.File.Delete(dirPath + fileURl + cirfileName + "." + cirextn);
}

FileStream fs1 = System.IO.File.Create(dirPath + fileURl + cirfileName + "." + cirextn);

Request.InputStream.CopyTo(fs1);

//stu_imageMBA = new Byte[Request.ContentLength];
//fs.Read(stu_imageMBA, 0, stu_imageMBA.Length);

//fs1.Write(stu_imageMBA, 0, stu_imageMBA.Length);
fs1.Close();

//Bitmap bt = new Bitmap(inputStream);
//bt.Save(context.Server.MapPath(dirPath + "/" + fileSave), ImageFormat.Jpeg);

//context.Session["Cat"] = true;
//return "{success:true, name:\"" + FileName + "\", path:\"" + Filepath + "\"}";
Session["IsSignFound"] = "1";
return "{success:true, imsg:\"Selected file is uploaded.\", name:\"" + filename + "\", path:\"" + fileURl2.Substring(1, fileURl2.Length - 1) + cirfileName + "." + cirextn + "\", spath:\"" + fileURl2 + cirfileName + "." + cirextn + "\", filetype:\"" + Request.ContentType + "\"}";
}
else
{
//return "Image size can not exceed " + size.ToString() + "kb in ";
return "{success:false, imsg:\"Image size can not exceed 100 kb in Signature file.\"}";
}
}
}
else
{
//return "Please upload .jpg or .jpeg file in ";
return "{success:false, imsg:\"Please upload .jpg or .jpeg file in Signature.\"}";
}
}
else
{
//return "Please upload .jpg or .jpeg file in ";
return "{success:false, imsg:\"Please upload .jpg or .jpeg file in Signature.\"}";
}


//f1.CircularURL = fileURl + fileName + extn;
//f1.CircularFileName = fileName;
//f1.CircularContentType = uploadFile.ContentType;

//myfile.Add(f1);
}
catch (Exception ex)
{
return "{success:false, imsg:\"Selected file is not uploaded. Please try again.\"}";
}
}


return "{success:false, imsg:\"Selected file is not uploaded. Please try again.\"}";