How to manage files in edit mode ASP.NET MVC Entity Framework?

Question

I am working on an ASP.NET MVC web application that uses Entity Framework.

Here is the model class:

public partial class Complaint
{           
    public int ComplaintId { get; set; }
    public string Reference { get; set; }
    public string ClientName { get; set; }
    public string AccountNo { get; set; }
    public Nullable<System.DateTime> DateComplaint { get; set; }
    public Nullable<decimal> Value { get; set; }
    public Nullable<int> TypeId { get; set; }
    public string Comment { get; set; }
    public Nullable<int> StatusId { get; set; }
    public Nullable<System.Guid> InsertedBy { get; set; }
    public Nullable<System.DateTime> CreatedDateTime { get; set; }
    public Nullable<System.DateTime> ModifiedDateTime { get; set; }  

    public virtual ICollection<FileAttach> FileAttaches { get; set; }
}

Controller method:

[Authorize]
[AuthorizeRoles("Admin", "Oficial")]
public ActionResult RequestApproval(int? id)
{
            ViewBag.Status = _context.Status.ToList();
            //ViewBag.User = _context.Profiles.Where(x => x.User.UserRoles.FirstOrDefault().RoleId == 2 || x.User.UserRoles.FirstOrDefault().RoleId == 3).ToList();

            if (id == 0)
            {
                return new HttpStatusCodeResult(HttpStatusCode.BadRequest);
            }            

            Complaint c = _context.Complaints.Include(s => s.FileAttaches).SingleOrDefault(x => x.ComplaintId == id);

            if (c == null)
            {
                return HttpNotFound();
            }

            return View(c);
}

[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult RequestApproval(int? id, HttpPostedFileBase upload)
{
            var c = _context.Complaints.Find(id);

            if (TryUpdateModel(c, "", new string[] { "Reference", "ClientName", "AccountNo", "DateComplaint", "Value", "TypeId", "Comment", "StatusId", "IsApproved", "ApprovedBy", "InsertedBy", "UpdatedBy", "CreatedDateTime", "ModifiedDateTime" }))
            {               
                for (int i = 0; i < Request.Files.Count; i++)
                {
                    upload = Request.Files[i];

                    if (upload != null && upload.ContentLength > 0)
                    {
                        Stream str = upload.InputStream;
                        BinaryReader Br = new BinaryReader(str);
                        Byte[] FileDet = Br.ReadBytes((Int32)str.Length);
                        var fileName = Path.GetFileName(upload.FileName);

                        FileAttach fileDetail = new FileAttach()
                        {
                            Name = fileName,
                            ContentType = upload.ContentType,
                            Data = FileDet,
                            AttachId = new int(),
                            Path = Path.Combine(Server.MapPath("~/Uploads/"), fileName),
                            ComplaintId = id
                        };

                        var path = Path.Combine(Server.MapPath("~/Uploads/"), fileDetail.Name);
                        upload.SaveAs(path);                        
                        _context.Entry(fileDetail).State = EntityState.Added;
                    }
                }

                _context.Entry(c).State = EntityState.Modified;
                _context.SaveChanges();

                return RedirectToAction("List");
            }

            return View(c);
}

My view:

<div data-parsley-validate class="form-horizontal form-label-left">
                @using (Html.BeginForm("RequestApproval", "Complaint", null, FormMethod.Post, new { enctype = "multipart/form-data" }))
                {
                    @Html.AntiForgeryToken()
                    @Html.ValidationSummary(true, "", new { @class = "text-danger" })
                    @Html.HiddenFor(model => model.ComplaintId, new { @class = "id" })

                    <div class="form-group">
                        <label class="control-label col-md-3 col-sm-3 col-xs-12" for="first-name">
                            @Html.LabelFor(model => model.Reference) <span class="required">*</span>
                        </label>
                        <div class="col-md-6 col-sm-6 col-xs-12">
                            @Html.DropDownListFor(model => model.StatusId, new SelectList(ViewBag.Status, "StatusId", "Denomination"), "- Please select a type -",
                                                  new { @class = "form-control" })
                        </div>
                    </div>
                    <div class="form-group">
                        <label class="control-label col-md-3 col-sm-3 col-xs-12" for="first-name">
                            @Html.LabelFor(model => model.Comment) <span class="required">*</span>
                        </label>
                        <div class="col-md-6 col-sm-6 col-xs-12">
                            @Html.TextAreaFor(model => model.Comment, new { @class = "form-control col-md-7 col-xs-12", style = "width:100% auto; height:150px;" })
                        </div>
                    </div>
                    <div class="form-group">
                        <label class="control-label col-md-3 col-sm-3 col-xs-12" for="first-name">
                            Files
                        </label>
                        <div class="col-md-6 col-sm-6 col-xs-12">
                            <input type="file" name="file" id="fileUpload" multiple="multiple" />
                            <ul class="attachment">
                                @foreach (var item in Model.FileAttaches)
                                {
                                    <li>
                                        <a class="title" href="/Complaint/Download/?p=@(item.Name)&d=@item.Name">@item.Name</a>
                                    </li>
                                }
                            </ul>
                        </div>
                    </div>
                    <div class="ln_solid"></div>
                    <div class="form-group">
                        <div class="col-md-6 col-sm-6 col-xs-12 col-md-offset-3">
                            <button type="submit" class="btn btn-primary btn-sm">Back To List</button>
                            <button type="submit" class="btn btn-success btn-sm">Save <i class="fa fa-save"></i></button>                                
                        </div>
                    </div>
                }
            </div>

When I try to edit I get this error:

Server Error in '/' Application
Exception Details: System.ArgumentNullException: Value cannot be null.
Parameter name: model

enter image description here