Data annotation reference not working issue

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C#

MVC5

I have a web application developed in MVC. It's an existing application. I want to validate the excel file with data annotation. But When I was trying to add the data annotation namespace in model class. I can only see system.componentmodel.design I am not able to find the data annotation under componentmodel namespace. Also HttpPostedFileBase is not working under system.web namespace. I removed the reference and added again but its not working. Please help me to solve this.

What I have tried:

using System;
using System.Web;
using System.Collections.Generic;
using System.Linq;
using System.ComponentModel.DataAnnotation;

namespace SMARTCON.Models
{
    public class UploadFileValidation
    {
        //[ValidateFile(ErrorMessage = "Please select a PNG image smaller than 1MB")]
        //public HttpPostedFileBase File { get; set; }

        [Required(ErrorMessage = "Please select file to upload")]
        [RegularExpression(@"([a-zA-Z0-9\s_\\.\-:])+(.xls|.xlsx)$", ErrorMessage = "Only .Xlx files allowed.")]
        public HttpPostedFileBase PostedFile { get; set; }
    }
}

[Required(ErrorMessage = "Please select file to upload")] this is coming as normal string only not as keyword