How to validate uploaded file in ASP.NET MVC?

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A custom validation attribute is one way to go:

public class ValidateFileAttribute : RequiredAttribute
{
    public override bool IsValid(object value)
    {
        var file = value as HttpPostedFileBase;
        if (file == null)
        {
            return false;
        }

        if (file.ContentLength > 1 * 1024 * 1024)
        {
            return false;
        }

        try
        {
            using (var img = Image.FromStream(file.InputStream))
            {
                return img.RawFormat.Equals(ImageFormat.Png);
            }
        }
        catch { }
        return false;
    }
}

and then apply on your model:

public class MyViewModel
{
    [ValidateFile(ErrorMessage = "Please select a PNG image smaller than 1MB")]
    public HttpPostedFileBase File { get; set; }
}

The controller might look like this:

public class HomeController : Controller
{
    public ActionResult Index()
    {
        var model = new MyViewModel();
        return View(model);
    }

    [HttpPost]
    public ActionResult Index(MyViewModel model)
    {
        if (!ModelState.IsValid)
        {
            return View(model);
        }

        // The uploaded image corresponds to our business rules => process it

        var fileName = Path.GetFileName(model.File.FileName);
        var path = Path.Combine(Server.MapPath("~/App_Data"), fileName);
        model.File.SaveAs(path);

        return Content("Thanks for uploading", "text/plain");
    }
}

and the view:

@model MyViewModel

@using (Html.BeginForm("Index", "Home", FormMethod.Post, new { enctype = "multipart/form-data" }))
{
    @Html.LabelFor(x => x.File)
    <input type="file" name="@Html.NameFor(x => x.File)" id="@Html.IdFor(x => x.File)" />
    @Html.ValidationMessageFor(x => x.File)
    <input type="submit" value="upload" />
}

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