MVC3 How to check if HttpPostedFileBase is an image

Question

I have a controller like this:

public ActionResult Upload (int id, HttpPostedFileBase uploadFile)
{
....
}

How can I make sure that uploadFile

Answer 1

You could check the file name and extension and MIME type but that might not be reliable because the user could simply rename the file before uploading. Here's a reliable way to achieve that by looking at the contents of the file: https://stackoverflow.com/a/6388927/29407

You could of course extend this to other known image type formats than PNG, like this:

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
        {
            var allowedFormats = new[] 
            { 
                ImageFormat.Jpeg, 
                ImageFormat.Png, 
                ImageFormat.Gif, 
                ImageFormat.Bmp 
            };

            using (var img = Image.FromStream(file.InputStream))
            {
                return allowedFormats.Contains(img.RawFormat);
            }
        }
        catch { }
        return false;
    }
}