error Object reference not set to an instance of an object

public class uploadImageMahsolat
    {
        [FileTypes("jpg,jpeg,png")]
        [Required(ErrorMessage = "its  Required.")]
        public HttpPostedFileBase Fileimg { get; set; }


        [FileTypes("jpg,jpeg,png")]
        [Required(ErrorMessage = "its  Required .")]
        public ICollection<HttpPostedFileBase> FileUpload900 { get; set; }

    }

public class FileTypesAttribute : ValidationAttribute
   {
       private readonly List<string> mTypes;

       public FileTypesAttribute(string _types)
       {
           mTypes = _types.Split(',').ToList();
       }

       public override bool IsValid(object _value)
       {
           if (_value == null)
           {
               return true;
           }




           var fileExt = System.IO.Path.GetExtension((_value as HttpPostedFileBase).FileName).Substring(1);





           return mTypes.Contains(fileExt, StringComparer.OrdinalIgnoreCase);
       }