ASP MVC Html.DisplayFileSize helper

DisplayFileSize.cs

namespace WhateverNamespaceYouWant
{
    using System;
    using System.Collections.Generic;
    using System.Configuration;
    using System.Linq;
    using System.Linq.Expressions;
    using System.Text;
    using System.Web;
    using System.Web.Mvc;
    using System.Web.Mvc.Html;
    using System.Web.Routing;

    public static class HtmlHelperExtensions
    {
        /// <summary>
        /// Display a human-readable number of bytes with at title for traditionalists.
        /// </summary>
        /// <param name="html"></param>
        /// <param name="length"></param>
        /// <param name="asHtml"></param>
        /// <param name="nullValue"></param>
        /// <returns></returns>
        public static IHtmlString DisplayFileSize(this HtmlHelper html, long? length, bool asHtml = true, string nullValue = null)
        {
            // see https://en.wikipedia.org/wiki/Binary_prefix
            // see https://physics.nist.gov/cuu/Units/binary.html
            if (length != null)
            {
                const long KiB = 1024, MiB = KiB * 1024, GiB = MiB * 1024, TiB = GiB * 1024;
                const long KB = 1000, MB = KB * 1000, GB = MB * 1000, TB = GB * 1000;

                string sign = string.Empty;
                if (length < 0D)
                {
                    sign = "-";
                    length = -length;
                }

                double sizeIEC = length.Value;
                double sizeMetric = length.Value;
                string suffixIEC, suffixMetric, longSuffixIEC, longSuffixMetric;
                string format = "{0:F2}";

                if (length >= TiB)
                {
                    sizeIEC = Math.Round((double)length / TiB, 2);
                    sizeMetric = Math.Round((double)length / TB, 2);
                    suffixIEC = "TiB";
                    suffixMetric = "TB";
                    longSuffixIEC = "tebibyte";
                    longSuffixMetric = "terabyte";
                }
                else if (length >= GiB)
                {
                    sizeIEC = Math.Round((double)length / GiB, 2);
                    sizeMetric = Math.Round((double)length / GB, 2);
                    suffixIEC = "GiB";
                    suffixMetric = "GB";
                    longSuffixIEC = "gibibyte";
                    longSuffixMetric = "gigabyte";
                }
                else if (length >= MiB)
                {
                    sizeIEC = Math.Round((double)length / MiB, 2);
                    sizeMetric = Math.Round((double)length / MB, 2);
                    suffixIEC = "MiB";
                    suffixMetric = "MB";
                    longSuffixIEC = "mebibyte";
                    longSuffixMetric = "megabyte";
                }
                else if (length >= KiB)
                {
                    sizeIEC = Math.Round((double)length / KiB, 2);
                    sizeMetric = Math.Round((double)length / KB, 2);
                    suffixIEC = "KiB";
                    suffixMetric = "kB";
                    longSuffixIEC = "kibibyte";
                    longSuffixMetric = "kilobyte";
                }
                else
                {
                    suffixIEC = "B";
                    suffixMetric = "B";
                    longSuffixIEC = "byte";
                    longSuffixMetric = "byte";
                    format = "{0:F0}";
                }

                var sb = new StringBuilder();
                sb.Append("<span class=\"FileSize\" title=\"");
                sb.Append(sign);
                sb.AppendFormat(format, sizeIEC);
                sb.Append(" ");
                sb.Append(longSuffixIEC);
                sb.Append(" | ");
                sb.Append(sign);
                sb.AppendFormat(format, sizeMetric);
                sb.Append(" ");
                sb.Append(longSuffixMetric);
                if (length.Value > KiB)
                {
                    sb.Append(" | ");
                    sb.Append(sign);
                    sb.Append(length.Value);
                    sb.Append(" bytes"); 
                }

                sb.Append("\">");
                sb.Append(sign);
                sb.AppendFormat(format, sizeIEC);
                sb.Append(" ");
                sb.Append("<abbr title=\"");
                sb.Append(longSuffixIEC);
                sb.Append("\">");
                sb.Append(suffixIEC);
                sb.Append("</abbr>");
                sb.Append("</span>"); 
                return MvcHtmlString.Create(sb.ToString());
            }
            else
            {
                return MvcHtmlString.Create(nullValue);
            }
        }
    }
}

It looks like this: