Utilise the MimeType validator

ValidateMimeType.cs

private bool ValidateMimeType(HttpPostedFile postedFile)
{
    bool valid = false;
 
    try
    {
        Stream stream = postedFile.InputStream;
 
        var mimeTypeUtil = new MimeTypeUtil();
        string mime = mimeTypeUtil.GetMimeType(ReadFully(stream), postedFile.FileName);
 
        valid = PermittedMimeTypes.Contains(mime);
    }
    catch (Exception ex)
    {
        Log.Error("Error occurred when determining mime type of file", ex, this);
        throw;
    }
 
    return valid;
}
 
private static byte[] ReadFully(Stream input)
{
    byte[] buffer = new byte[16 * 1024];
    using (MemoryStream ms = new MemoryStream())
    {
        int read;
        while ((read = input.Read(buffer, 0, buffer.Length)) > 0)
        {
            ms.Write(buffer, 0, read);
        }
        return ms.ToArray();
    }
}
 
private IEnumerable<string> _permittedMimeTypes;
private IEnumerable<string> PermittedMimeTypes
{
    get
    {
        if (_permittedMimeTypes == null)
        {
            _permittedMimeTypes = new List<string>()
                    {
                        MimeType.ImageBmp,
                        MimeType.ImageGif,
                        MimeType.ImageJpeg,
                        MimeType.ImagePng,
                    };
        }
 
        return _permittedMimeTypes;
    }
}