Validate File Extensions

Question

I'm trying to validate that only .jpg file types can be uploaded into my local file storage and then include the URL of that location in the database.

I would like to check for the validation on each of the files that I'm trying to upload and if either file does NOT include .jpg then I would like to halt the application from moving forward with a message stating "Invalid file format".

I have the following code below. At this time I'm not getting any error messages, but rather the page will just refresh without displaying an error or message and it will also not proceed with the correct file format.

Controller

[HttpPost]
    [ValidateAntiForgeryToken]
    public IActionResult Upsert(DepartmentPageCreateVM departmentPageVM, List<IFormFile> files)
    {
        if (ModelState.IsValid)
        {
            if (departmentPageVM.DepartmentPage.DepartmentPageId == 0)
            {
                _context.DepartmentPages.Add(departmentPageVM.DepartmentPage);
            }
            else
            {
                _context.DepartmentPages.Update(departmentPageVM.DepartmentPage);
            }
            _context.SaveChanges();

            string wwwRootPath = _webHostEnvironment.WebRootPath;
            if (files != null)
            {
                foreach (IFormFile file in files)
                {
                    //string productPath = @"images\departments\department-" + departmentPageVM.DepartmentPage.DepartmentPageId;
                    string fileName = Guid.NewGuid().ToString() + Path.GetExtension(file.FileName);
                    string productPath = @"images/departments/department-" + departmentPageVM.DepartmentPage.DepartmentPageId;
                    string finalPath = Path.Combine(wwwRootPath, productPath);

                    if (!Directory.Exists(finalPath))
                        Directory.CreateDirectory(finalPath);

                    // check file extension
                    string extFile = Path.GetExtension(fileName);
                    if (extFile != ".jpg")
                    {
                        ModelState.AddModelError("", "Invalid file format");
                    }

                    using (var fileStream = new FileStream(Path.Combine(finalPath, fileName), FileMode.Create))
                    {
                        file.CopyTo(fileStream);
                    }

                    DepartmentPhoto departmentPhoto = new()
                    {
                        //DepartmentPhotoImageURL = @"\" + productPath + @"\" + fileName,
                        DepartmentPhotoImageURL = @"/" + productPath + @"/" + fileName,
                        DepartmentPageId = departmentPageVM.DepartmentPage.DepartmentPageId,
                    };

                    if (departmentPageVM.DepartmentPage.DepartmentPhoto == null)
                        departmentPageVM.DepartmentPage.DepartmentPhoto = new List<DepartmentPhoto>();

                    departmentPageVM.DepartmentPage.DepartmentPhoto.Add(departmentPhoto);
                }
                _context.DepartmentPages.Update(departmentPageVM.DepartmentPage);
                _context.SaveChanges();
            }
            return RedirectToAction("Index");
        }
        else
        {
            departmentPageVM.DepartmentList = _context.Departments.ToList().Select(u => new SelectListItem
            {
                Text = u.DepartmentName,
                Value = u.DepartmentId.ToString()
            });
            return View(departmentPageVM);
        }
    }

File Input in View

<div class="form-group mt-4">
            <label asp-for="DepartmentPage.DepartmentPhoto"></label>
            <input type="file" name="files" class="form-control" multiple />
        </div>

ViewModel

public class DepartmentPageCreateVM
{
    public DepartmentPage DepartmentPage { get; set; }

    [ValidateNever]
    public IEnumerable<SelectListItem> DepartmentList { get; set; }
}

DepartmentPhoto Model

public class DepartmentPhoto
{
    public int DepartmentPhotoId { get; set; }
    public string? DepartmentPhotoImageURL { get; set; }

    public int DepartmentPageId { get; set; }
    public DepartmentPage? DepartmentPage { get; set; }
}

DepartmentPage Model

public class DepartmentPage
{
    public int DepartmentPageId { get; set; }
    public string Name { get; set; }
    public string ContentBody { get; set; }

    public int DepartmentId { get; set; }

    [ValidateNever]
    public Department Department { get; set; }

    [ValidateNever]
    public List<DepartmentPhoto> DepartmentPhoto { get; set; }
}

Answer 1

From the code I think you need to return the view with model after adding the ModelState.AddModelError("", "Invalid file format"); in order to display the error.

You can also do saveChanges() after all images uploaded without any error as per best practice but you can ignore this advice if your requirement is different.

And better way to validate any file is :

1. check content type of file.
2. check the extension
3. load the file in memory and check initial bits.

For image you can refer the code of this answer : Determine if uploaded file is image (any format) on MVC