How to validate uploaded file in ASP.NET MVC?

I have a Create action that takes an entity object and a HttpPostedFileBase image. The image does not belong to the entity model.

I can save the entity object in the database and the file in disk, but I am not sure how to validate these business rules:

• Image is required
• Content type must be "image/png"
• Must not exceed 1MB

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][1] [1]: http://codeanalyze.com/Articles/Details/6/MVC-5-Validate-Posted-Files-view-models-viewmodels

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();
    } 
    //....
    //...
}

And file length validation in asp.net core:

public async Task<IActionResult> MyAction()
{
    var form = await Request.ReadFormAsync();
    if (form.Files != null && form.Files.Count == 1)
    {
        var file = form.Files[0];
        if (file.Length > 1 * 1024 * 1024)
        {
            ModelState.AddModelError(String.Empty, "Maximum file size is 1 Mb.");
        }
    }
    // action code goes here
}

You may want to consider saving the image to database also:

  using (MemoryStream mstream = new MemoryStream())
                {
                    if (context.Request.Browser.Browser == "IE")
                        context.Request.Files[0].InputStream.CopyTo(mstream);
                    else
                        context.Request.InputStream.CopyTo(mstream);

                    if (ValidateIcon(mstream))
                    {
                        Icon icon = new Icon() { ImageData = mstream.ToArray(), MimeType = context.Request.ContentType };
                        this.iconRepository.SaveOrUpdate(icon);
                    }
                }

I use this with NHibernate - entity defined:

 public Icon(int id, byte[] imageData, string mimeType)
    {
        this.Id = id;
        this.ImageData = imageData;
        this.MimeType = mimeType;
    }

    public virtual byte[] ImageData { get; set; }

    public virtual string MimeType { get; set; }

Then you can return the image as a FileContentResult:

 public FileContentResult GetIcon(int? iconId)
	{
        try
        {
            if (!iconId.HasValue) return null;

            Icon icon = this.iconRepository.Get(iconId.Value);

            return File(icon.ImageData, icon.MimeType);
        }
        catch (Exception ex)
        {
            Log.ErrorFormat("ImageController: GetIcon Critical Error: {0}", ex);
            return null;
        }
	}

Note that this is using ajax submit. Easier to access the data stream otherwise.