Update an existing parent entity with multiple child entities in database ASP.NET MVC

Question

I want to update my existing entities that consist a parent entity and two child entities.

Previously I have utilized view model to create data that include a parent class and two child class, however I could not use view model as parameter in update because it requires an integer parameter to know which Id I should update.

Relationship diagram:

Model Relationship

Model classes:

public class Requisition
{
    [Key]
    public int requisitionId { get; set; }

    public string Branch { get; set; }

    public List<TravelAttachment> AllFiles { get; set; }
    public List<Itinerary> Itineraries { get; set; } = new List<Itinerary>();
}

public class TravelAttachment
{
    [Key]
    public int AttachId { get; set; }
    public string FilePath { get; set; }
    public int? requisitionId { get; set; }
}

public class Itinerary
{
    [Key]
    public int ItineraryId { get; set; }
    public string Departure { get; set; } = "";
    public string Arrival { get; set; } = "";
    public int? requisitionId { get; set; }
}

View model:

public class RequisitionViewModel
{
    public int requisitionId { get; set; }

    public string Branch { get; set; } = "";

    public List<IFormFile> File { get; set; }
    public List<Itinerary> Itineraries { get; set; } = new List<Itinerary>();
}

What I have tried is to create a view model object in my controller, and the outcome is telling me one of my variable in view model object get returned with null value.

Controller

public IActionResult UpdateRequisition(int id)
{
        if (ModelState.IsValid)
        {
            var requisition = _context.Requisitions
            .Include(r => r.Itineraries)
            .Include(r => r.AllFiles)
            .AsNoTracking()
            .FirstOrDefault(m => m.requisitionId == id);

            string uniqueFileName = null;

            // have null object here
            RequisitionViewModel model = new RequisitionViewModel();

            List<TravelAttachment> attachments = new List<TravelAttachment>();

            //problem that inform my model.File is null 
            foreach (IFormFile file in model.File)
            {
                    string uploadsFolder = Path.Combine(_webHostEnvironment.WebRootPath, "dist/files");

                    uniqueFileName = Guid.NewGuid().ToString() + "_" + file.FileName;

                    string filePath = Path.Combine(uploadsFolder, uniqueFileName);

                    file.CopyTo(new FileStream(filePath, FileMode.Create));

                    attachments.Add(new TravelAttachment { FilePath = uniqueFileName });

            }

            // using automapper to link viewmodel with model
            var requisitionApplied = _mapper.Map<Requisition>(model);

            requisitionApplied.AllFiles = attachments;
            requisitionApplied.Itineraries = requisition.Itineraries;

            _context.Requisitions.Update(requisitionApplied);
            _context.SaveChanges();

            return RedirectToAction("Index");                 
        }

        return View(id);
} 

In this case, I am not able to upload new files in this action, what should I do to make sure user can upload new files?

Please ask me for additional information if you need more elaboration. I am appreciated for your kind support!