var ext = System.IO.Path.GetExtension("C://soar.jpg");

private static Dictionary<String, ImageFormat> GetImageFormats()
{
    var dic = new Dictionary<String, ImageFormat>();
    var properties = typeof(ImageFormat).GetProperties(BindingFlags.Static | BindingFlags.Public);
    foreach (var property in properties)
    {
        var format = property.GetValue(null, null) as ImageFormat;
        if (format == null) continue;
        dic.Add(("." + property.Name).ToLower(), format);
    }
    return dic;
}

private static Dictionary<String, ImageFormat> _imageFormats;
/// <summary>
/// 獲取 所有支持的圖片格式字典
/// </summary>
public static Dictionary<String, ImageFormat> ImageFormats
{
    get
    {
        return _imageFormats ?? (_imageFormats = GetImageFormats());
    }
}

/// <summary>
/// 根據圖像獲取圖像的擴展名
/// </summary>
/// <param name="image"></param>
/// <returns></returns>
public static String GetExtension(Image image)
{
    foreach (var pair in ImageFormats)
    {
        if (pair.Value.Guid == image.RawFormat.Guid)
        {
            return pair.Key;
        }
    }
    throw new BadImageFormatException();
}

using (var img = Image.FromFile(@"C:/soar"))
{
    var ext = GetExtension(img);
}