Creating custom validation attribute in MVC

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MVC

Hi, I need to upload a pdf file on server so I am using

<input type="file" name="InvoiceFile" id="FileField1" size="38" />

for this. But somewhere I saw fileupload is not accessible through javascript for security reasons. Now only one option left (as I think) creating custom validation attribute. so I wrote following code in Model

public class ValidateFileAttribute : RequiredAttribute
   {
       public override bool IsValid(object value)
       {

               var file = value as string;
               if (file == null)
               {
                   return false;
               }
               else
               {
                   return true;
               }

       }
   }

and in model calss

[ValidateFile(ErrorMessage = "You must upload file before you proceed.")]
        public HttpPostedFileBase File { get; set; }

in view

<input type="file" name="InvoiceFile" id="FileField1" size="38" />
                                        @Html.ValidationMessageFor(x => x.File)

in controller

 if (!ModelState.IsValid)
                {
                    return RedirectToAction("Work_orders", "Contractor");
                }
// rest of code

while I debug the code I get null in object value parameter of

public override bool IsValid(object value)

of custom attribute class even after uploading file. so its always returning false and I am unable to pass validations, any solution.
Thanks