Uploading files of perticular extension.

if (fileUploadCategoryImage.HasFile == true)
              {
                  categoryImageName = fn_UploadCategoryImage(fileUploadCategoryImage);
              }

public string fn_UploadCategoryImage(FileUpload UploadedFile)
    {
        HttpPostedFile file = UploadedFile.PostedFile;
        string fileExt = Path.GetExtension(file.FileName).ToLower();
        string fileName = Path.GetFileName(file.FileName);
        if (fileName != string.Empty)
        {
            try
            {
                if (fileExt == ".jpg" || fileExt == ".gif" || fileExt == ".bmp" || fileExt == ".jpeg" || fileExt == ".png")
                {

                    str = DateTime.Now.ToFileTime() + "_" + fileName;
                    file.SaveAs(Server.MapPath("~/admin/Upload/Category/") + str);

                }
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }
        //str = fileName;
        return str;
    }