Check if uploaded file is an image in C# ASP.NET MVC - asp.net-mvc

Question

I have my controller
[HttpPost]
public ActionResult ChangeAvatar(HttpPostedFileBase file)
{
AvatarHelper.AvatarUpdate(file, User.Identity.Name);
return RedirectToAction("Index", "Profile");
}
And I already check if file is in jpeg/png format:
private static bool IsImage(string contentType)
{
return AllowedFormats.Any(format => contentType.EndsWith(format,
StringComparison.OrdinalIgnoreCase));
}
public static List<string> AllowedFormats
{
get { return new List<string>() {".jpg", ".png", ".jpeg"}; }
}
What I need - it ensure that uploaded file is real image file and not txt file with image extension.
I convert my uploaded file like this:
using (var image = System.Drawing.Image.FromStream(postedFile.InputStream))
{
///image stuff
}
I am thinking about try/catch block on creating image from input stream but I wonder if there is good way to do it?
Thanks)
P.S.
I wonder if there is another (more efficient way that try/catch block) way to check whether file is real image?

Answer 1

You could use the RawFormat property:
private static ImageFormat[] ValidFormats = new[] { ImageFormat.Jpeg, ImageFormat.Png };
public bool IsValid(Stream image)
{
try
{
using (var img = Image.FromStream(file.InputStream))
{
return ValidFormats.Contains(img.RawFormat);
}
}
catch
{
return false;
}
}
Also you could put this validation logic into a reusable validation attribute as I have shown in this post.

Answer 2

My solution as an extension, actually checking if a base64 string is an image or not:
public static bool IsImage(this string base64String)
{
byte[] imageBytes = Convert.FromBase64String(base64String);
var stream = new MemoryStream(imageBytes, 0, imageBytes.Length);
try
{
stream.Write(imageBytes, 0, imageBytes.Length);
System.Drawing.Image image = System.Drawing.Image.FromStream(stream, true);
return true;
}
catch (Exception)
{
return false;
}
}
Usage:
if(!"base64string".IsImage())
throw new Exception("Not an image");