That's pretty funny the FileUpload control's contentType can lie to you. I guess it just loads the file into a byte[] and let's you take it from there.

I think you can test the real file contents information by loading the file into a System.Drawing.Image and testing it's RawFormat property. There, you will find a Guid that is unique for each image type. If you can't load the file into an Image, I guess it's not a jpg, png or gif as you requested.


System.Drawing.Image img = System.Drawing.Image.FromStream(FileUpload1.PostedFile.InputStream);

if (img.RawFormat.Guid == System.Drawing.Imaging.ImageFormat.Jpeg.Guid) {do something}

Or, if you want to return the type of the image, try this:

    public static string MimeType(System.Drawing.Image imgPhoto)
    {
        foreach (ImageCodecInfo codec in ImageCodecInfo.GetImageDecoders())
        {
            if (codec.FormatID == imgPhoto.RawFormat.Guid)
                return codec.MimeType;
        }
        return "image/unknown";
    }