Validate uploaded files extension in ASP.Net MVC

I am trying to validate file upload for mime type and other validations, i am uploading three different type of formats, pdf, zip and ppt, so please tell me how can i check whether the file is a valid pdf not any text file with just extension change. 

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
using System.Linq;
using System.Web;

namespace OnlineStationaryRegister.Models
{
    public class TourDetail
    {
        [Key]
        public int FileId { get; set; }
        public string Officername { get; set; }
        public string Designation { get; set; }
        public string TourPdfPath { get; set; }
        public string TourPPtPath { get; set; }
        public string TourZipPath { get; set; }
        public string Tourpdffilename { get; set; }
        public string Tourpptfilename { get; set; }
        public string Tourzipfilename { get; set; }
        public int status { get; set; } = 1;


        [NotMapped]
        [Required]
        [Display(Name ="Tour PDF")]
        [RegularExpression(@"([a-zA-Z0-9\s_\\.\-:])+(.doc|.docx|.pdf)$",ErrorMessage ="Only PDF Files Allowed")]
        public HttpPostedFileBase tourpdfupload { get; set; }
        [NotMapped]
        [Required]
        [Display(Name = "Tour PPt")]
        public HttpPostedFileBase tourpptupload { get; set; }
        [NotMapped]
        [Required]
        [Display(Name = "Tour zip")]
        public HttpPostedFileBase tourzipupload { get; set; }
    }
}

public ActionResult Create()
        {
            return View();
        }

       
        [HttpPost]
        [ValidateAntiForgeryToken]
        public ActionResult Create([Bind(Include = "FileId,Officername,Designation,TourPdfPath,TourPPtPath,TourZipPath,Tourpdffilename,Tourpptfilename,Tourzipfilename,status")] TourDetail tourDetail)
        {
            if (ModelState.IsValid)
            {
                db.TourDetails.Add(tourDetail);
                db.SaveChanges();
                return RedirectToAction("Index");
            }

            return View(tourDetail);
        }

@model OnlineStationaryRegister.Models.TourDetail

@{
    ViewBag.Title = "Create";
}

<h2>Create</h2>


@using (Html.BeginForm("Create", "Tour", FormMethod.Post, new { @enctype = "multipart/form-data" }))
{
                @Html.AntiForgeryToken()

                <div class="form-horizontal">
        <h4>TourDetail</h4>
        <hr />
        @Html.ValidationSummary(true, "", new { @class = "text-danger" })
        <div class="form-group">
            @Html.LabelFor(model => model.Officername, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.EditorFor(model => model.Officername, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model => model.Officername, "", new { @class = "text-danger" })
            </div>
        </div>

        <div class="form-group">
            @Html.LabelFor(model => model.Designation, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.EditorFor(model => model.Designation, new { htmlAttributes = new { @class = "form-control" } })
                @Html.ValidationMessageFor(model => model.Designation, "", new { @class = "text-danger" })
            </div>
        </div>

        <div class="form-group">
            @Html.Label("Upload PDF", htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.TextBoxFor(model => model.tourpdfupload, new { type = "file" })
                @Html.ValidationMessageFor(model => model.tourpdfupload, "", new { @class = "text-danger" })
                @*<input type="file" name="tourpdfupload" />*@
            </div>
        </div>

        <div class="form-group">
            @Html.Label("Upload PPT", htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.TextBoxFor(model => model.tourpptupload, new { type = "file" })
                @Html.ValidationMessageFor(model => model.tourpptupload, "", new { @class = "text-danger" })
            </div>
        </div>

        <div class="form-group">
            @Html.Label("Upload Zip", htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                @Html.TextBoxFor(model => model.tourzipupload, new { type = "file" })
                @Html.ValidationMessageFor(model => model.tourzipupload, "", new { @class = "text-danger" })
            </div>
        </div>

        

        <div class="form-group">
            <div class="col-md-offset-2 col-md-10">
                <input type="submit" value="Create" class="btn btn-default" />
            </div>
        </div>
    </div>
}

<div>
    @Html.ActionLink("Back to List", "Index")
</div>

@section Scripts {
    @Scripts.Render("~/bundles/jqueryval")
}