How to validate uploaded file in ASP.NET MVC?

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" />
}

Based on Darin Dimitrov's answer which I have found very helpful, I have an adapted version which allows checks for multiple file types, which is what I was initially looking for.

public override bool IsValid(object value)
    {
        bool isValid = false;
        var file = value as HttpPostedFileBase;

        if (file == null || file.ContentLength > 1 * 1024 * 1024)
        {
            return isValid;
        }

        if (IsFileTypeValid(file))
        {
            isValid = true;
        }

        return isValid;
    }

    private bool IsFileTypeValid(HttpPostedFileBase file)
    {
        bool isValid = false;

        try
        {
            using (var img = Image.FromStream(file.InputStream))
            {
                if (IsOneOfValidFormats(img.RawFormat))
                {
                    isValid = true;
                } 
            }
        }
        catch 
        {
            //Image is invalid
        }
        return isValid;
    }

    private bool IsOneOfValidFormats(ImageFormat rawFormat)
    {
        List<ImageFormat> formats = getValidFormats();

        foreach (ImageFormat format in formats)
        {
            if(rawFormat.Equals(format))
            {
                return true;
            }
        }
        return false;
    }

    private List<ImageFormat> getValidFormats()
    {
        List<ImageFormat> formats = new List<ImageFormat>();
        formats.Add(ImageFormat.Png);
        formats.Add(ImageFormat.Jpeg);
        formats.Add(ImageFormat.Gif);
        //add types here
        return formats;
    }
}

Here is a way to do it using viewmodel, take a look at whole code here

Asp.Net MVC file validation for size and type Create a viewmodel as shown below with FileSize and FileTypes

public class ValidateFiles
{
    [FileSize(10240)]
    [FileTypes("doc,docx,xlsx")]
    public HttpPostedFileBase File { get; set; }
}

Create custom attributes

public class FileSizeAttribute : ValidationAttribute
{
    private readonly int _maxSize;

    public FileSizeAttribute(int maxSize)
    {
        _maxSize = maxSize;
    }
    //.....
    //.....
}



public class FileTypesAttribute : ValidationAttribute
{
    private readonly List<string> _types;

    public FileTypesAttribute(string types)
    {
        _types = types.Split(',').ToList();
    } 
    //....
    //...
}