Checking MIME Type from a base64 string I've written the following extension to determine the MIME type of a base64 string.  It's worked in my local tests, but can anyone point out issues or alternate methods?

public static AttachmentType GetMimeType(this string value)
{
    if(String.IsNullOrEmpty(value))
        return new AttachmentType
                {
                    FriendlyName = "Unknown",
                    MimeType = "application/octet-stream",
                    Extension = ""
                };

    var data = value.Substring(0,5);

    switch (data.ToUpper())
    {
        case "IVBOR":
        case "/9J/4":
            return new AttachmentType
                {
                    FriendlyName = "Photo",
                    MimeType = "image/png",
                    Extension = ".png"
                };

        case "AAAAF":
            return new AttachmentType
                {
                    FriendlyName = "Video",
                    MimeType = "video/mp4",
                    Extension = ".mp4"
                };
        case "JVBER":
            return new AttachmentType
                {
                    FriendlyName = "Document",
                    MimeType = "application/pdf",
                    Extension = ".pdf"
                };

        default:
            return new AttachmentType
                {
                    FriendlyName = "Unknown",
                    MimeType = string.Empty,
                    Extension = ""
                };
    }
}

public class AttachmentType
{
    public string MimeType { get; set; }
    public string FriendlyName { get; set; }
    public string Extension { get; set; }
}
Your switch is wrong: "/9J/4" is for JPG images, not PNG (e.g. as shown in these examples).